Try to guess the correct order of the blocks in order to make the program below work (hint: try to deduce from the even parity of the parentheses and the indentation):

This study material is the so-called lecture notes for the course Programming 1. The phrase "lecture notes" refers to literary material that introduces the topics of the course approximately in the same order and the same perspective as they are introduced on lectures. In order to keep the length of the lecture notes in check, topics are not introduced in exhaustive detail. Therefore, a good book on the topic and an unprejudiced attitude towards searching for information on your own are recommended to support your learning. The most current information is of course found online - as long as you remember source criticism. Note also that most of the books that are available approach programming from the point of view of a certain programming language - especially those directed towards beginners. This is somewhat natural, because people also need a common language to communicate with each other. For the same reason, learning to program would be quite difficult without learning the basics of one language first.

To make the structure clearer, most books usually cover one topic systematically from the beginning to the end. However, when a child is learning how to speak, they are not able to adopt all of the grammar rules of a certain clause structure all at once. Similarly, while learning the basics of programming, the student may not yet have the necessary understanding to be able to grasp all structures and possibilities. Discussion concerning the topics in these lecture notes as well as during lectures is arranged as follows: first, we give examples or discuss the necessity of these examples. Then, we explore the theoretical and practical aspects of the topic. Hence, these lecture notes stratch the surface of the basics of programming from one point of view. Books and online sources offer necessary advanced information.

The language of choice in these lecture notes is the C# language. It should be noted, however, that the chosen language should be treated as just an example: the structure and examples in the lecture notes could be almost identical for any other programming language. The most important thing on an introductory course to programming is to learn the programmer's way of thinking. Changing the language to another language in the same language family is more like changing dialects in a natural language rather than changing languages altogether. In other words, if you learn to program in one language, you can already read programs coded in other languages after a small amount of practice. Coding in another language may be more challenging, but usually they contain the same constructs. Programming languages come and go, and you shouldn't learn just one, but learn as many as you can. Programming courses equivalent to this one have been taught in the University of Jyväskylä with the following languages: Fortran, Pascal, C, C++, Java, and now C#. Some universities use Python, others use Scala as their beginner language. All of these in some sense belong in the same language family and abide mostly by the same principles even though the details may vary a lot from time to time.

It is impossible to learn how to program by just reading books. For this reason, the course also contains weekly exercises (demos), supervised computer lab work, and a programming project. More information about these, as well acquiring and installing the tools used on the course can be found on the course home page:

https://tim.jyu.fi/view/kurssit/tie/ohj1/koti

These lecture notes are based on the Programming 1 lecture notes written in 2009 by Martti Hyvönen and Vesa Lappalainen, which was the result of the work by multiple different authors starting all the way from the 80's. The largest contributions have been made by Timo Männikkö and Vesa Lappalainen.

Jyväskylä, 2.1.2013

Martti Hyvönen, Vesa Lappalainen, Antti-Jussi Lakanen

The newest version of the lecture notes have been written directly in TIM (The Interactive Material). The idea behind TIM is that different things, such as programming, can be tried without installing any programs at all. This will hopefully lower the threshold to starting programming. Unfortunately the technology we use (the language and subroutine libraries) do not offer the possibility to make interactive games in TIM, so for more advanced programming we still need to install programming tools, Visual Studio and Jypeli in this case. These tools will be discussed later in these lecture notes and in other materials for the course in more detail.

Special thanks to the ACOS Content Server project in the Aalto University for the algorithm visualisations in the material.

Jyväskylä, 29.8.2014 Vesa Lappalainen, Antti-Jussi Lakanen

The 2023 version of the brochure will change the references to Visual Studio to a more generic one, because in the JY courses the tool has been changed to Rider.

0. Introduction

Even though the course is designed as a "game programming" course, 90% of its content is exactly the same as in any other programming course. If there is someone who does not want to make a game as one's project work, it is of course possible to write any other kind of small program.

0.1 About the content and aims of the course

To get the general idea (in English) of the content of this course, watch the video about how to make the game GalaxyTrip in less than five minutes. While watching these videos, don't be afraid of the fact that you can't do the same (yet); Watch these videos in order to understand what you should learn and will learn during this course.

If you want to watch a longer (45 minutes) video about the same topic (in Finnish), watch this video:

These videos showcase what kinds of games have been made in courses before:

0.2 Aims of the course

At the beginning of the course, you are expected to be able to know how to use a computer. You should be familiar with at least the use of different (text) editors, keyboard shortcuts, and preferably the command prompt as well. These days, of course, the command prompt is (unfortunately) not very familiar to most, which is why it is recommended that you see the extra material page of this course or Paavo's survival guide (Note: both of these are currently available only in Finnish; if you cannot understand Finnish, see for example An A-Z Index of the Windows CMD command line or search for English command prompt tutorials)

You won't need any previous programming experience.

Remember to view the video directory of the course as well.

0.3 TIM instructions

This TIM-environment-based material consists of different interactive parts. Videos were already introduced in the previous chapters. It is best to close the videos after viewing to save the capacity of your device.

This material contains links to other materials. These links are extra material and there is no need to follow them when going through the material for the first time. It is easy to get lost in a jungle of links.

It is best to be logged in (Login) at all times in order to keep track of your own progress. When you are logged in, you can see red bars on the right side of the material which indicate the parts that you haven't marked as read yet. When you have read (and understood :-) ) a passage of text, click the red bar to remove it. This way you can easily see which parts you haven't gone through yet. This is especially helpful if you jump from place to place in the material in different order than in which it was written. The bar may also be yellow when you have read a passage, but its contents have changed since you've read it. Click on the yellow bar after you have understood and internalised the changed passage.

The upper left corner of the page contains an image of a book or depending on the size of the screen/resolution a menu image which contains an image of a book. Click on the book image to open the table of contents. Click the image again to close the table of contents.

When you point the cursor to the left side of a paragraph, a blueish green bar appears. Clicking this bar opens a menu where you can find e.g. the Comment/Note button, which allows you to add notes related to the paragraph. Use this feature actively. You can add notes either to yourself or use a comment to point out that something in the paragraph is unclear or erroneous.
In case of an error in the paragraph, please provide a correction suggestion as well. Otherwise, use the "Everyone" choice with consideration, and add your personal notes with the choice "Just me".

If you want to search for something, use the Find function of your browser (Ctrl-F in most browsers).

If you want to find a specific page again easily, make a TIM bookmark of it. You can add a bookmark by clicking the paper clip icon in the upper left corner of the page. Of course you can also add a regular bookmark in your browser, but the benefit of a TIM bookmark is that it works in any browser. Start by bookmarking this page. Click the paper clip icon and add the page for example under the heading Ohj1 with the name Material or Handout.

In the preceding videos, programs were coded in the Visual Studio IDE (Integrated Development Environment). TIM itself contains a small built-in environment, which can complete simple tasks, such as:

There is a link Show all code under the task box. By clicking this link you can view all of the code required to run this program. You can still edit the program, but you cannot insert any code in the "wrong" place. The same link also hides the "extra code".

The Highlight link changes to a type of editor that highlights the code according to the syntax of the language and can fill in words that are familiar to the editor.

The link Reset allows you to "reset" your answer and start from template from the beginning. Try each link.

The task could continue by adding (before the line Add(ball)) the line:

       ball.Position = new Vector(150, 100);

Try this as well. So, copy/paste the line of code above to the longer code from earlier, above the line Add(ball). See what happens when you write the colour Red in lower case. Correct the colour back to capitalised Red and see what happens when you change the values in Vector.

You can also play around with this example coded in another language (VPython). Here, you can also change the point of view by holding the right mouse button and moving the mouse around.

1. What is programming?

1.1 Algorithms and instructions

At its simplest, programming is giving instructions in order to perform a predefined task. There are many actions similar to programming in a person's everyday life. An example of an algorithm is giving directions to someone by phone so that they can drive to a place formerly unfamiliar to them. We do this by giving a series of instructions and commands that direct the performance of this action. Nowadays a navigator reads directions one by one. Similarly, a program executes commands one by one, keeping track of the place where the execution currently is. An example of rudimentary programming is using a microwave oven; the oven is given instructions of how long it should operate and how much power to use.

Programming has many levels. Nowadays there are, for example, tractors where the farmer can program it to go through the fields in a certain pattern. For caution and in case of sharp turns, the farmer must still be inside the tractor to make sure that everything goes well. In a certain way, the farmer must also know how to program. But before we got to this point, it required a vast amount of engineering work and programming to make sure that the GPS satellites, error correction and the programming in the tractor's computer got to the level where it is easy for the farmers themselves to program.

In Suonenjoki, strawberry pickers have an NFC (Near Field Communication) chip on their necks, so that every time a picker has filled his/her punnet and takes it to the gathering point, it is automatically registered how many kilos of strawberries were picked, who picked them, and where they picked them. The farmer has programmed in the information about the location of the fields and the actions and can now monitor more efficiently which field has decreased profit and should be "formatted" altogether.

To sum up, computers and programming are present everywhere in everyday life. However, very often the users are not even aware (and hopefully don't need to be) that they are using a computer and perhaps even programming it.

In these cases, the concept we are dealing with is embedded systems, and/or IoT (Internet of Things) if the device is connected to the web, like in the case of the tractor and farmer.

So, the previous examples concerned giving unambiguous instructions. However, these examples included very different types of communication situations. Interpersonal communication, turning the switches and pressing the buttons of a microwave oven, and timing a digital television adapter are all parallel when it comes to programming, but they include the use of different tools. In programming, the choice of tools depends on the devices or tools available for performing the task at hand. Interpersonal communication can happen via talking, writing, or a combination of the two. Similarly, programming often allows choosing between different methods of implementation, depending on the nature of the task.

Although programming is for the most part performed on a computer, it is always good to have a pen and paper on hand. The most difficult part of programming for a beginner is that the person often can't bear to sit down with a pen and paper and think about what needs to be done. If for example you are designing a Battleship game, you first need to play the game multiple times in order to get an idea of all the possible things that you will face while designing the game.

Programming has multiple levels depending on which tools are used for completing the task at hand. Advanced high-level tools allow working with concepts and expressions which, at their best, are similar to concepts and expressions in a natural language, whereas low-level tools require working with simple and rudimental concepts and expressions.

This recipe for making a sponge cake can be considered an example of programming:

Sponge cake

6       eggs
1,5 dl  sugar
1,5 dl  flour
1,5 tsp baking powder

1.  Whip the eggs and sugar into a froth.
2.  Mix the flour and baking powder.
3.  Blend the egg-sugar froth and flour mixture together.
4.  Bake for 45 mins in 175°C

The recipe if clearly written for a person, more specifically for someone who knows quite a lot about baking. If the same recipe was written for a person who has never baked anything in his/her life, the recipe above would not be even nearly sufficient - it would have to include many tips related to baking: pre-heating the oven, the art of whipping ingredients to a froth, etc.

Instructions written for a machine differ considerably from the instructions written for a person. A machine doesn't automatically know how to ask for advice when it runs into a new and unexpected situation. It operates exactly according to the instructions it has been given, whether or not they are sensible in the current situation. Unlike people, a machine loyally repeats the instructions it has been given without succumbing to creativity. Because of this, programming languages today require presenting the instructions to a machine in a very precise format and taking into account all the possible emerging situations. [MÄN]

1.2 On programming languages

This section is a brief introduction to the basic principle of computers and programming languages. For more information on the topic, refer to the course materials for Computer Organization and Architecture and Operating Systems. The topic is also discussed briefly in chapter Lukujen esitys tietokoneessa.

1.2.1 Processor and machine code

The most important parts of the computer are the processor and memory. An essential feature of the processor is that it knows which command to run. The command is usually stored in the IP register (Instruction Pointer, also known as PC = Program Counter). The IP register points to a memory slot that holds the command to run. The processor actually works in a simple way:

1. Get the command from the memory slot indicated by the IP register
2. Increment the IP register so that it points to the next command
3. Run the command (can change the IP with JUMP commands)
4. Continue from 1.

Registers are fast memory slots inside the processor. Commands are usually low-level, e.g.

• retrieve number from memory slot 7F34 to the AX register
• Add the value of BX register to the AX register

Each command has a numeric value in the form of bits. For example the command

• put number 62 (hex) in BL register

in an Intel x86 processor would be in form

B3 62

and in memory in binary form

10110011 01100010

To sum up, the programming task here would be to enter the correct binaries to the memory of the computer. Because binaries are hard to read by humans, they are usually presented as hexadecimals. Even that is not trivial, remembering that B3 means "put in BL register". This is why an assembly language is usually used: it has almost a 1:1 equivalence with machine language binary and human-readable mnemonic. In one assembly language (there are many variants) the previous command would be

mov bl,$62

First, computers were programmed by entering command numerals directly. Then with the advent of assembly languages, commands were typed in assembly and compiled to numerals in order to get the correct program in the computer's memory.

Because processor commands are rudimentary, a lot of them are needed to make up even the simplest of programs. Especially when reading user input or a file. This is why an operating system is needed: to offer the most frequently used features out-the-box. Nevertheless, writing even a small program in an assembly language would require a lot of lines of code.

Since the 1950s, programming languages were developed to make writing programs easier and clearer than with assembler. Fpr example, Fortran (1957), Lisp (1958), Cobol (1959), and Pascal (1970) are still used today. By the 1970s, there were already dozens, even hundreds of languages when including all the small languages.

1.2.2 C language and robot

A language compiler is a program that reads as input a human-written plain-language (e.g. C or C++ -language) program file (text file) and produces a binary executable machine-language file, which can then be run. This is why, for example, in a Windows system the name of the executable file is often specified as .exe. When the program is started, the operating system's task is to put the program code into the computer's memory and pass the program counter to the first instruction of the program.

C language (1972) is the most known of all compiled high level languages developed in the 1970s. Its idea (as with its predecessors) is to raise the abstraction to a higher level, so that we can for example state:

int a = 15;
int b = 23;
int c = a + b;

If this was written in machine language, the programmer would have to know which memory slots to use for storing variables a, b, and c. In C programs (and C# programs used on this course) the compiler takes care of used memory slots, e.g. all references to variable a are compiled by the compiler to a reference to the memory slot reserved for a.

The course exercises have as example a small robot that knows only a few commands. The robot works similarly to a processor. For example the previous part of a C program (which is exactly the same in C#) would be:

You can try the robot by clicking the Step button. As an exercise, you can change it to count the sum of all numbers on the Input line (however, this requires that the 0 on the line stops counting). You can put more numbers on the Input line by placing them in Preset input and clicking Reset. Click Run to get the robot to run the entire program.

The yellow line on Program is equivalent to the processor's IP register which points to the command to run.

The used language is basically the robot's assembly language.

If commands are given numeric values (which they actually have internally), for example:

00 = INPUT
01 = OUTPUT
02 = ADD
03 = SUB
04 = COPYTO
...
09 = JUMPIFNEG

this program would be compiled to the robot's "machine language" like this:

00 04 00 00 02 00 01

where some of the commands take up two bytes (a byte is eight bits, presented in two number pairs), for example COPYTO which has the command's numeric value and the target address for the command (memory slot 00 here).

Then we could have a C compiler that would compile the previously described program to these numbers. Memory slots a and b would be compiled to places on the Input line. The same program could also be made using memory slots:

This is already very similar to a carefully written C program. One of the tasks of the compiler would be to decide that references to variable a mean memory slot 00 and references to b mean memory slot 01.

1.2.3 Byte languages

C language was the majority language from late 70s to late 80s. During the 80s, a backward compatible language, extended with objects was made: C++(1982). It was also a compiled language. In the 90s, Java was developed (1995), originally as a language for different embedded systems. Java also mended some known problems with C++. Java had some significant differences compared to C++:

1. Java is not compiled directly to machine language, but into an intermediate language. The intermediate language file is run with a processor-specific program called Java. The Java program (Java Virtual Machine) reads the intermediate byte code (cf. robot language in numeric format) and executes it step by step. Java was not the first byte-code-based language, but it is currently the most widely known virtual-machine-based language.
2. Java has automatic memory allocation, meaning that the programmer doesn't need to remember to free the memory slots that have been reserved. Automatic memory allocation already existed before Java.
3. With Java, it is not possible to point to a memory slot that has not been reserved for this purpose (i.e. there are no pointers in Java).

The principle of byte code is that there is no need to make a compiler for each processor architecture and operating system separately. One compiler is enough to generate the intermediate language file (usually .class in Java). On the other hand, executing the program requires interpreting the intermediate language to actual processor machine language, and at first Java programs were slower than C programs. Nowadays Java compilers are more developed, and byte code is also compiled to machine language while it is executed (JIT = Just In Time compiling), meaning that if the same code is executed again, it has already been compiled and executing speed is no different than with C code.

Popularity of Java skyrocketed in the latter half of the 90s. VL's opinion on why this happened: >"The reason is automatic memory allocation that lowered the number of errors that programmers usually made. Java also had functional strings, which were missing from the C++ standard at the time. The C-like syntax also significantly lowered the threshold to switch languages."

Microsoft had put a lot of effort into C++, but they also noticed the popularity of Java and started using it, adding their own features into it. This cause licensing disputes with Sun, the company that developed Java. Microsoft then started developing its own language which would have all the good qualities of Java. The resulting language was C# (C sharp, 2000). The languages were very similar, making it easy for programmers to switch between them.

1.2.4 C# and Jypeli

Around 2008-2009, the Faculty of Information Technonology in University of Jyväskylä started planning a programming course suitable for young people. It was quite clear that it would contain making games. At the time, Microsoft had great environments (Visual Studio) and libraries (XNA) for making games with C# and get them to work on both computers and phones (Windows Phone). However, programming games with XNA was too challenging, which is why Jypeli library was developed to hide away "extra" details that would hinder a beginner's enthusiasm. The resulting course, "Nuorten pelikurssi" was a success. At the same time, university students had problems with motivation on Java-based programming courses. Many university students are also interested in games, which is why the theme of the first programming course was switched to game programming, also changing the tools to Jypeli and C#. Making "more meaningful" programs significantly increased the pass rate of Programming 1. Just printing "Hello World" no longer piques interest in the 2010s.

1.2.5 Other languages

A couple of popular languages were mentioned above, C, C++, Java, and C#. They basically share the same roots. Intermediate language interpreting was also mentioned. One very well-known, originally interpreted language is Basic (1964). Its idea was that compiling stage is omitted and the program code is executed directly row by row. Nowadays Python (1991) is a very popular interpreted language. A different approach to programming is functional programming, which can be done with for example Haskell (1990), Scala (2004), and F# (2005).

Javascript is a language used in browsers that makes the originally static HTML pages "come alive". For example, this material is viewed on an application called TIM, where a server program written in Python and Haskell transmits Javascript (1995) and HTML (1993), which allows the browser to form an interactive text. Additionally, nowadays TIM is written with TypeScript (2012) instead of Javascript, which is then compiled into Javascript. In 3D graphics, shader languages like GLSL and HLSL are used regardless of which languages other parts of the application using the graphics is written in. In addition to these, there are countless different DSLs (domain specific languages). In summary, writing one program may require knowing multiple different programming languages.

• Try different programming languages in TIM
• HelloWorld in different languages, example contains 603 programming languages

The popularity and history of different languages can be read about in the links below. The popularity of languages can be measured in multiple ways, so take different indices with a pinch of salt.

• Tiobe-index

Nevertheless, the example language on this course is C#.

2. First C# program

2.1 Coding a program

C# (pronounced c sharp) programs can be coded with any text editor. There are dozens, even hundreds of text editors, so naming just one for coding purposes is hard. However, some have been specifically designed for programming. These kinds of text editors can automatically format the source code (or just code in short) that the programmer produces so that reading is easier and therefore understanding and editing it faster. Programmers favour editors such as Vim, Emacs, Visual Studio Code, Sublime Text and NotePad++, but other text editors are presumably as good as these. Any text editor is applicable to coding the examples at the beginning of this material.

Next, write a C# program similar to the example below and save it for example under the name HelloWorld.cs. The established filename extension is .cs, which stems from the name of the used programming language and which we will in this course as well. Be careful when you save a file because some text editors save files with the extension .txt by default, and in this case the file name can accidentally become HelloWorld.cs.txt.

This program should print the text

Hello World!

In order to try out the functionality of this program, it must first be compiled into a form that the computer can understand.

When you click the Aja('Run') button in TIM, the program is first transformed into machine language format and if the compilation is successful, the program is run and its output is printed on the screen. More information about these phases is presented in the sections to follow. Before that, here are some tasks where you can test your skills, so to say.

Examples of the HelloWorld program coded in different programming languages can be found e.g. here:

http://www2.latech.edu/~acm/HelloWorld.html.

Try what happends and why if you leave away all letters \n.

2.2 Compiling and running programs

In order to compile and run a program, a C# application developer has to be installed on the computer. If you use Windows, a sufficient starting tool is Microsoft .NET SDK (Software Development Kit). With other operating systems, one of the options is Novell Mono. Many of the exercises on this course can be completed with the Mono application developer, but the instructions and examples in the material are for the Windows development environment. Once again, the use of all the features of the Jypeli library is only possible in the Windows environment.

For example, the TIM environment that you are using is implemented (in Python, Haskell, and Javascript) so that the text you write is relayed to a Linux server which saves it in a temporary file and compiles it with the aforementioned Mono compiler. If the compilation is completed without errors, the created assembly language program is run on the Linux server and the output is captured and shown on the browser window. All these phases only take a few seconds.

Lisätietoa .NET-kehitystyökaluista ja asentamisesta löytyy kurssin kotisivuilta kohdasta Työkalut.

Next, we will learn to complete these phases manually in order to gain a better understanding of what happens in the background.

Kääntäjän versiot vaihtuvat helposti vuosittain, samoin miten niitä käytetään. Ajantasaisimman esimerkin kääntämisestä löydät harjoituksesta:

• Pääteohjaus 1, HelloWorld (syksy 2023)

The program should print the text Hello World! on the screen, as in the image below.

2.3 The structure of the program

Although the "only significant line" in our first program is

the C# language requires surrounding information about which part of the program contains this statement and from which part the program should start. This adds a little to the number of code lines in this program that in itself is simple. In some languages, it is enough to simply write the print statement. Generally, a low number of lines has no intrinsic value, which is why the (low) number of lines is not enough to define which language is the best.

The HelloWorld.cs program we coded (or actually the text file that we wrote) is almost the simplest possible C# program. Below are the two lines of the simplest possible program.

public class HelloWorld
{

The first line defines the class, the name of which is HelloWorld. At this stage, it is enough to think of the class as a "home" for subroutines (aka subprograms). Subroutines will be discussed later. On the other hand, a class can also be compared to a "cookie cutter" - it is a building instruction for creating objects (or "cookies"). During the run-time of a program, objects can be created with the code inside a class. Objects can also be destroyed. One class can be used to create many objects of the same type, exactly like one cookie cutter can be used to make a lot of (almost) identical cookies.

Each C# program contains at least one class, but there can be multiple classes as well. The name of the class that contains the program code is preferably the same as the name of the file. If the name of the file is HelloWorld.cs, it is recommended that the name of the class is also HelloWorld, just like in the example. At this stage, it is not absolutely necessary to fully understand what a class exactly is - this will become clearer later.

The word public before the word class is a type of an access modifier. The access modifier allows the class either to be displayed without limitations or with some limitations to others (classes) or to be hidden completely from others. The word public means that the class is public from the point of view of other classes, as most classes usually are. Other access modifiers include protected, internal, and private.

The access modifier can also be blank, which will automatically define the class as internal. Subroutines will be discussed later, but it can be mentioned that accordingly, if the access modifier of a subroutine is left blank, it automatically becomes private. However, in this course we will practice coding public classes (and subroutines), in which case the word public will almost always be written in front of the class and subroutine. Notice however that when we will discuss object variables (attributes), their access modifier will almost without exception be private.

The second line contains a left brace {. In many programming languages, parts that belong together are grouped or gathered within braces. A left brace is an opening brace, which in this case tells the compiler that this is where the contents of the HelloWorld class start. For every opening brace there must be a closing right brace }. The ending brace of the HelloWorld class is in line five, which is also the last line of the program. The area between the braces is called a block.

Line three defines (or more specifically introduces) a new subroutine named Main. Because it uses this name, it is the main program of the class. The words static and void are always a part of the introduction of the Main subroutine. static means that the subroutine is class-specific (as opposed to object-specific, in which case the word static is not used). void means that the subroutine does not return any information. These modifiers will be discussed in detail later. Main may also return a value, in which case the word void is replaced with int, but this feature will not be used during this course.

Similarly to classes, the contents of the main program are also coded within braces. In C# the execution of program code always starts from the main program (Main) of the executed class. Of course, many things happen on the background even before this.

On line four, the text Hello World! is printed on screen. In C# this is accomplished by requesting the .NET environment's Console class in the System class library to print the text using the WriteLine() method.

Libraries, objects, and methods will be discussed more in section 4.1 and in chapter 8.

The character string to be printed is written in quotes inside parentheses (Shift + 2). This line is also the only statement in the program. Statements can be thought of as being individual actions that form the program. In C#, each statement ends with a semicolon. Because a semicolon marks the end of a statement, white spaces, such as line breaks or spaces, make no difference to the function of a program in the syntax of C#. They are highgly relevant to the readability of the code, however. Note that forgetting to add a semicolon is one of the most common errors in programming, or more specifically one of the most common syntax errors.

Note that in the example below the value of variable a is printed by forming a new string which combines another string and value of a with a plus operator. In this way, we can give the WriteLine subroutine just one string as parameter as is supposed to. The WriteLine subroutine cannot be given a list with a comma separator like in some languages.

2.3.1 Error types

Programming errors can be broadly categorised into syntax errors and logical errors.

In the previous example, it was studied where extra spaces or line breaks are allowed. When the program could not be compiled, it was due to a syntax error. When the program worked, but the output looked different, it was due to an error in writing style (or a difference in opinion).

A syntax error prevents the compilation of the program even if the meaning (aka the semantics) of the program was correct. Examples of syntax errors include spelling mistakes or forgetting to add a semicolon at the end of a statement.

Logical errors happen when the semantics, aka the meaning, of the program is faulty. They are harder to notice because the program is compiled successfully despite semantic errors. The program may even seem to be working exactly as intended. If a logical error goes unnoticed even during testing, the consequences can be catastrophic, depending on the purpose of the software. Here is one commonly known example of a logical error which was luckily noticed in time and correcting it prevented a wide-spread disaster:

https://en.wikipedia.org/wiki/Y2K.

2.3.2 Interpreting compiler error messages

The following is an example of a syntax error in the HelloWorld program.

The program contains a minor spelling error, which (without aid) is fairly hard to notice. Study the error message given by the csc compiler.

HelloWorld.cs(5,17): error CS0117: 'System.Console' does not
contain a definition for 'Writeline'

The compiler tells us that row 5 and column 17 in the file HelloWorld.cs contains the following error: 'System.Console' does not contain a definition for 'Writeline'. This is true, because the word Line in WriteLine should be capitalised. After fixing this spelling error, the program works.

Unfortunately, the content of the error message does not always describe the problem very well. In the following example, a semicolon has been misplaced. Try to find it yourself first before continuing or running the program.

The error message, or error messages look something like this, depending on the compiler:

HelloWorld.cs(4,3): error CS1519: Invalid token '{' in class,
struct, or interface member declaration
HelloWorld.cs(5,26): error CS1519: Invalid token '(' in class,       
struct, or interface member declaration
HelloWorld.cs(7,1): error CS1022: Type or namespace definition,
or end-of-file expected

The first error message points to line 4, when in reality the stem of the problem is in line 3. In other words, the compiler error messages are not helpful in this case; on the contrary, they instruct us to do something we don't want to, and should not, do.

More examples of interpreting error messages can be found on the extra material for the course (page currently available in Finnish only).

2.3.3 White spaces

As we tried out in a task earlier, the HelloWorld example program could be coded, without altering its function, in the following alternative format.

Furthermore, the program could also be coded as follows.

Or even by writing all of the code on one line, try it.

Although both of the examples above are syntactically correct, in other words, they abide by the grammatical rules of C#, their readability is significantly lower than in the original program. C# has commonly agreed coding conventions that define how program code should be written. When everyone writes code in the same way, it is easier to read the code. The examples in this material are coded according to these conventions. Links to coding conventions can be found on the course extra information page (currently only available in Finnish):

https://tim.jyu.fi/view/kurssit/tie/ohj1/materiaali/koodauskaytanteet

A string of text is written between " quotes. While processing strings, spaces, tabs, and line breaks are significant. Compare the output of the programs below.

The line above prints out:

Hello World!

whereas the line below prints out:

H e l l o    W o r l d !

To make reading easier, white spaces are used in front of lines to indent blocks. It is customary that after each opening brace the code lines are indented by four spaces, and the indentation is decreased accordingly by four spaces after a closing brace. In C#, a pair of braces is usually located in the same column. Usually IDEs automatically format the text, which is definitely a feature worth using if you don't know how to format the text properly yourself.

2.4 Commenting

“Good programmers use their brains, but good guidelines save us having to think out every case.” -Francis Glassborow

The C# language contains three different types of comments and four different types of annotation:

Source code is usually hard to understand just from reading the code itself. This is why you can and you should add descriptions or comments in your code. Comments help both the programmer himself/herself as well as any person who reads or maintains the code in the future. Many things may seem obvious at the time of writing, but even after a week you may wonder what purpose each line of your code serves.

The compiler ignores comments, so they won't affect the functionality of the program.

// Single-line comment

Single-line comments start with two slashes (//). They're effective until the end of the row.

/* This comment
   lasts for
   multiple
   lines */

A comment that starts with a slash and an asterisk (/*) lasts until another asterisk and slash ends it (*/). Note that there is no space between the asterisk and slash.

For example, the comment line /* cat */ can be added in the same places where spaces could be added in a previous task. Respectively, you cannot write a comment line in places where you cannot add spaces either.

2.4.1 Documentation

The third comment type is the documentation comment. Documentation comments have a specific syntax, and abiding by its rules allows transforming documentation comments into a summary, which can be viewed in a browser or printed neatly on paper, for example.

A documentation comment should be written in front of each class, main program, subroutine, and method (subroutines and methods will be discussed later). In addition, each C# file should start with a documentation comment that clarifies the meaning, author, and version of the file.

Documentation comments always start with three slashes (Shift + 7). Accordingly, each following documentation comment line starts with three slashes as well.

/// This
/// is
/// a documentation comment

Documentation utilises tags. If you have ever written HTML pages, this type of notation should be familiar to you. Documentation comments begin with a start tag, like <example>, which is followed by the content of the comment. Comments end with an end tag, like </example>, i.e. otherwise just like the start tag but with one slash after the first angle bracket.

C# tags include for example <summary>, which is a short summary of the block of code to follow (for example the main program or a method). The summary ends with the end tag </summary>.

During compilation, documentation tags can be saved in a separate XML file, from which they can be easily transformed into browsable HTML pages. You can come up with more tags, but the list of recommended tags is sufficient for the purposes of this course. Information of the recommended tags can be found in the C# documentation:

http://msdn.microsoft.com/en-us/library/5ast78ax.aspx

We can now add the following C# comments to the beginning of the HelloWorld program:

The author is given at the beginning of the program. This is followed by the first documentation comment (note the three slashes), which is a brief description of the class. Nore that in some documentation summaries only this first sentence is shown. Click the link Document and study the generated documentation by clicking the links in it. The content of the documentation is gathered from the program's documentation comments that start with ///.

Documentation comments ensure that a program could eventually be documented to the similar extend as Jypeli.

Documentation is an extremely important part of programming. As the number of classes and lines of code grows, documentation eases both your own work as well as the work of future users and administrators. The importance of documentation is emphasised by the fact that as much as 40-60% of the work time of administrators is spent on trying to understand how the program under revision works. [KOSK][KOS]

3. Algorithms

“First, solve the problem. Then, write the code.” - John Johnson

3.1 What is an algorithm?

When we write machine-readable instructions, the action to be performed needs to be written as a series of simple actions. This series of actions needs to be unambiguous, which means that in each situation it can offer only one course of action, and it cannot contain any contradictions. An unambiguous description of the series of actions that need to be taken in order to perform a task are called an algorithm.

The coding process of a program can start with outlining the necessary algorithms, i.e. by listing the necessary actions to perform a task:

Brewing coffee:

1.  Fill the coffee pot with water.
2.  Boil the water.
3.  Add the coffee grounds.
4.  Let the coffee even out.
5.  Serve the coffee.

In general terms, an algorithm is a series of actions defined as specifically as possible, consisting of unambiguous step-by-step actions that are necessary to solve the task at hand.

3.2 Specification

When inspecting almost any assignment, you will notice that the performance of the task consists of clearly distinguishable parts of the task. The way in which a single part of the task is performed has no effect on performing the other parts. Only the fact that each part is performed has relevance to the result. For example, each part of the coffee brewing task can be divided into smaller parts:

Brewing coffee:

1. Fill the coffee pot with water:
  1.1.  Place the pot below the tap.
  1.2.  Open the tap.
  1.3.  Let the water run until there is enought water in the pot.
  1.4   Close the tap.
2.  Boil the water:
  2.1.  Place the pot on the stove.
  2.2.  Turn the hotplate on.
  2.3.  Let the hotplate warm up until the water boils.
  2.4   Turn the hotplate off.
3.  Add the coffee grounds:
  3.1.  Measure out the coffee grounds.
  3.2.  Mix the coffee grounds into the boiling water.
4.  Let the coffee even out:
  4.1. Wait until most of the coffee has blended into the water.
5.  Serve the coffee:
  5.1. This is a story on its own...

The solution to the coffee brewing problem was divided into five phases. The algorithm of the solution contains five statements to perform. Upon closer inspection, it turns out that each of these five statements is divisible to even smaller phases, i.e. the main algorithm of the solution can be divided into subalgorithms which present steps to solve each phase.

Writing algorithms turns out to be a hierarchical process, where a task is divided into parts which are specified until each part of the task is so simple that there is nothing ambiguous about it.

3.3 Generalisation

One important stage in writing algorithms is generalisation. Generalisation means trying to locate all the factors in an algorithm that depend on the task at hand and analysing if these factors could be replaced with more general factors or even removed completely.

3.4 Exercise

3.5 Sequence

Similar to the recipe in chapter 1 and other instructions written for people, also instructions for computers are read top-down, unless defined otherwise. For example, instructions for drawing a snowman could be presented in the simplified manner below.

Draw a circle of radius 20 cm to the point (20,80)
Draw a circle of radius 15 cm on top of the previous circle 
Draw a circle of radius 10 cm on top of the previous circle

The code above is not written in any programming language yet, but it already contains the idea of how to draw a snowman on a computer. We will draw a snowman in C# in the next chapter.

An example of an algorithm: let's assume that you face a situation where you have an array of numbers and each number in this array should have the same value as the first number in the array. In the following task, you can write an "algorithm" for this situation by using the Tauno program.

Drag the elements in the array in Tauno so that you finally have the required result. At the same time, look at what kind of code Tauno generates for you. This is an algorithm for performing this task in C#. If you want to start again, hide Tauno and show Tauno again (click Hide Tauno and Click here to show Tauno).

Try to think if the order of the statements in your solution to the Tauno task could be altered. If it could, your code is parallel; if it couldn't, your code is purely sequential.

4. A simple graphical C# program

The following example utilise the Jypeli programming library developed in University of Jyväskylä. Originally the library was designed and implemented for the Youth game programming course, but it was found suitable for the level of the Programming 1 course as well. You can download the library from here:

https://tim.jyu.fi/view/kurssit/tie/ohj1/tyokalut/tyokalut,

4.1 What is a library?

C# programs consist of classes. Classes contain methods (and subroutines/ functions), which perform tasks and possibly return values after performing these tasks. A method could, for example, calculate the sum of two numbers and return the result, or draw a circle of the requested size. Methods related to the same topics are collected into classes and classes are further collected into libraries. The idea behind libraries is that there is no need to redo something that has been done by someone else already. In other words, there is no need to reinvent the wheel.

The most significant library for a C# programmer is the .NET Framework Class Library (FCL). The documentation of the class library is worth exploring, because it contains multiple methods that are very useful. The documentation can be found on Microsoft's site at

https://learn.microsoft.com/fi-fi/dotnet/.

4.2 The Jypeli library

Development of the Jypeli library started in University of Jyväskylä in spring 2009. The examples in this material use version 4. The Jypeli library contains classes and methods which make it easier to include for example physical and mathematical phenomena as well as characters and their movements into your own programs.

4.3 Example: Snowman

The description that follows refers to the line numbers in this program.
Click the link Highlight to show the line numbers.

When you run the program, it should draw a simple snowman in the middle of the screen, as in the image below.

For continuation, we will shorten the program and write the repeated main program in a separate file. This way, we can focus on the problem itself. Try to add a fourth circle to the snowman.

4.3.1 Running a program

Execution of a program always starts from the opening brace of the main program and moves line by line, top-down all the way to the closing brace of the main program. The main program (just like any other subroutine) can also contain subroutine calls, which move the execution from the main program to the subroutine and then back to the main program (the subroutine that made the call). Subroutines will be discussed in detail in chapter 6. In fact, even the examples from before made subroutine calls, such as Add(p1)

Let's inspect the most significant parts of the Snowman program.

First we have to let the compiler know that we want to utilise the entire Jypeli library. Now all the classes in the Jypeli library (and their methods) are in our use. In fact, we don't even have to use the statement using. But if we leave it out, the compiler will not recognise words such as PhysicsGame. This problem could be solved by stating that it can be found in the Jypeli library:

And similarly, Jypeli. would have to be added in front of each introduction of a Jypeli tool. For this reason, we can save trouble by simply stating using Jypeli. In fact, if we would have stated this at the beginning of the HelloWorld.cs file:

for printing, it would have been enough to write:

Now, let's return to inspecting the Snowman program:

08 /// <summary>
09 /// A class where we practice drawing cirles on screen.
10 /// </summary>
11 public class Snowman : PhysicsGame
12 {

Lines 8-10 are documentation comments. Line 11 creates a new class, Snowman, which differs from the style of creating a class HelloWorld example. Here, we are using the Jypeli library for the first time to state that the Snowman class that we are creating is "based" on the PhysicsGame class in the Jypeli library. To be more specific, the Snowman class is inherited from the PhysicsGame class. This way the Snowman class can utilise all of the features in the PhysicsGame class and can add new features to it. Here, we are adding the function of the Begin method, which defines what will be drawn at the beginning of the "game". In a way, Begin is the "main program" of a Jypeli program.

Drawing and (later) moving objects on screen and utilising laws of physics is much easier by utilising the PhysicsGame class.

14   /// <summary>
15   /// The main program starts the "game" as is customary in Jypeli  
16   /// </summary>
17   public static void Main()
18   {
19     using (Snowman game = new Snowman())
20     {
21       game.Run();
22     }
23   }

Also the Main method, aka the main program, is practically always written in this format in Jypeli, so we won't have to change it much in the future. We will skip explaining the contents of the main program at this point and simply state that the main program creates a new object (i.e. a new "game") using the Snowman class which is then run with the statement game.Run(). Due to the structure of the Jypeli library, all of the actual game-related code is coded within their own subroutines. Next, the code that is executed at the beginning of a "game" is coded into the Begin subroutine, which will be executed next.

To be specific, Begin starts from line 29. The first statement is written on line 30.

The first of these two lines calls the ZoomToLevel subroutine within the Camera object, which makes sure that the "camera" is focused and zoomed in on the right spot. The subroutine doesn't require any parameters, so the area between the parentheses is blank. The second line changes the colour of the background.

Let it be known that the Camera and Level objects are objects of the game (the variable game in the main program) created from the Snowman class. In reality, the code should be as follows:

but when we refer to the object's own features, the self-reference this. can be left out. Some programmers like to write out the self-reference for clarity, although it is not necessary. This is a typical example of a matter of opinion in programming.

With these three lines, we create a new physics object, a circle, give its radius, y-coordinate, and add it to the "stage", i.e. the visible area of the program. If the x-coordinate is not provided, it is 0 by default.

More specifically, we create a new PhysicsObject object, i.e. an instance of the PhysicsObject class, which we name p1. PhysicsObject objects are objects that move on the game field and abide by the laws of physics. Within the parentheses, we state what kind of object we want to create - in this case, the width and height (on the Jypeli scale, not in pixels) and the shape of the object. In summary, we are creating a circle (Circle) of radius 100 (width = 2 * 100 and height = 2 * 100). Other shapes in the Shape collection include a triangle (Triangle), an ellipse (Ellipse), a rectangle (Rectangle), a heart (Heart), etc. Objects will be discussed in more detail in chapter 8.

The next line defines the position of the object with its Y-coordinate value:

Notice that Y is capitalised. This is an attribute of the p1 object. The x-coordinate need not be defined separately because it is 0 by default and that suits us. To draw circles in specific positions, we need to calculate the coordinates. By default, the centre of the window is the origin of the coordinates, i.e. the point (0, 0). The values of the x-coordinate grow when moving right and the values of the y-coordinates grow when moving up, similarly to "ordinary" coordinates that we learn in school.

The coordinates can also be entered in vector format by providing both of the coordinate components at the same time. For example, in the previous task the ball could have been placed in the position x=20,y=50 by coding:

The game object always needs to be added to the stage before it becomes visible. This can be done with the Add method, which takes in the name of the object to be added as its parameter (here p1).

More specifically, we should state that we are adding the object to this game, like this:

but as discussed before, self-references can be left out.

The information provided to methods is parameters. The method ZoomToLevel doesn't take in any parameters, but the Add method takes in one parameter: a PhysicsObject object that we want to add to the stage. Another parameter that the Add method can take in is the layer number to which the object is added. By using the layers you can manage which objects are added on top. The layer parameter can be left out of the method call though, which makes the program decide the best order of the layers by itself.

Parameters are written in parentheses after the method name and separated by commas.

MethodName(parameter1, parameter2,..., parameterN);

In the following lines we create two more circles in a similar way, but changing the radius and coordinates of the circles.

In the Snowman example, the arithmetic operations of C# are utilised in calculating the coordinates. Of course, we could calculate the coordinates ourselves, but why should we if the computer can do it for us? The basic arithmetic operations of C# are sum (+), subtraction (-), multiplication (*), division (\), and the remainder (%). Arithmetic operations will be discussed in more detail in section 7.7.1.

The middle circle is placed on top of the top circle so that the circles touch. In other words, the centre of the middle circle is located so that its x-coordinate is 0 and its y-coordinate is the position of the bottom circle + the radius of the bottom circle + the radius of the middle circle. If we want the radius of the middle circle to be 50, we need to place its centre in the position (0, p1.Y + 100 + 50), which can be drawn with the statement:

Notice that in addition to setting the Y-attribute of the physics object (set) we can also read or request (get) the value of the parameter in question. In the example above, we do this by simply writing p1.Y to the right side of the assignment operator =.

The following image demonstrates the positioning of the first and second ball.

The top circle touches the middle circle. As a practice assignment, calculate the coordinates of the top circle when its radius is 30.

All information about classes, class methods and what parameters each method takes in can be found in the documentation of the library you are using. The class documentation of Jypeli can be found here:

http://kurssit.it.jyu.fi/npo/material/latest/documentation/html/.

Documentation of Jypeli is in Finnish, but the names of classes and attributes are in English.

4.4 Exercise

Find the class RandomGen in the documention of the Jypeli library. What information can you find about the
NextInt(int min, int max) method?
What other methods does the class contain?

The next example teaches the use of the random number generator in the C# library. The example is run in the Mono environment and it gives different results for the random number from the .NET environment. The example demonstrates that the random number in the Mono environment does not work well. Both Jypeli and the Windows .NET environment have this problem with the random number fixed.

4.5 Compilation and referring to class libraries

In order to compile the Snowman example program with a C# compiler, you need to save the Jypeli library on your computer. Jypeli utilises not only the XNA library but also free open-source physics and mathematics libraries. In other words, the Jypeli library has built-in physics and mathematics libraries.

Before compiling in the command prompt, copy the following files from the course website (see: Snowman in command prompt(in Finnish)) and paste them in the same folder that contains the Snowman.cs file.

• Jypeli.dll (found in the jypeli.zip file)

We still need to tell the compiler to use the Jypeli library in order to compile the Snowman code. In addition, the compiler needs the information that this program is made for the use of 32-bit systems (x86). This can be done with the help of the /reference parameter of the csc program (the compiler). The reference to the XNA library that Jypeli utilises is needed as well. Write the following command (all rows on the same line, one space in front of the /) in the command prompt

csc Snowman.cs /reference:Jypeli.dll;
 "%XNAGSv4%\References\Windows\x86\Microsoft.Xna.Framework.Game.dll" 
 /platform:x86

Koska näin komennoista tulisi varsin pitkiä ja sitä varten Microsoft on tehnyt dotnet-nimisen ohjelman, jolla voidaan hallita näitä tarvittavien kirjastojen suhteita. Tämän ohjelman avulla kääntämisen vaiheet ovat seuraavat

1. Yhden kerran asennetaan Jypelin tarvitsemat kirjastot, eli annetaan komentoriviltä komento

   dotnet new install Jypeli.Templates 

   Tätä ei tarvitse enää antaa toista kertaa

2. Siirrytään luodaan tarvittaessa ja siirrytään hakemistoon, johon uusi projekti halutaan

   cd HAKEMISTOPOLKU    

3. Luodaan uusi projekti Lumiukkoa varten

   dotnet new Fysiikkapeli -n Lumiukko

4. Tässä syntyy Lumiukko-hakemistoon mm Lumiukko.cs niminen tiedosto, joka muokataan halutulla tavalla toimivaksi.

5. Käännetään ja ajetaan ohjelma

   dotnet run

6. Jos ei toimi halutulla tavalla, muokataan tiedostoa ja käännetään ja ajetaan uudelleen.

More information about the topic on the course's extra material page (in Finnish).

5. From source code to the processor

5.1 Compiling

Now, we will inspect more carefully how the C# source code eventually transforms into a format that the processor can understand. When the programmer creates the program's source code that utilisises the .NET Framework environment, the internal compilation process is divided into two phases. First, the program is compiled into an intermediate language, MSIL (Microsoft Intermediate Language), which is not yet executable in any operating system. During runtime, the finished program is compiled from this indermediary phase code to the desired operating system and processor. The operating system can be for example Windows, macOS, iOS, Android, or Linux. The processor can be for example one of the Intel processors abiding the x86 architecture or with mobile devides for example ARM. This run-time compilation is performed with the so-called JIT compiler (Just-In-Time). The JIT compiler transforms the intermediary code into code that is compatible with the operating system specifically during run-time - hence the name "just-in-time".

Before the first compilation the compiler checks that the code has the correct syntax. [VES][KOS]

Compilation was performed with the Windows command prompt by using the command

5.2 Executing/running the program

So, C# produces an executable (or "runnable") file from the source code. This file is OS-dependent and only executable on the platform on which the compilation was performed. In other words, programs that have been compiled in the Windows environment are not executable in macOS, or vice versa.

Unlike C#, some other programming languages produce OS-independent code. For example, in Java, the file that the compiler produces is in so-called bytecode, which is OS-independent. In order to execute Java bytecode, a Java Virtual Machine is required. A Java Virtual Machine is a program that imitates a real computer which interprets bytecode and executes it on the processor of the target computer. This differs significantly from traditional compiled languages (such as C and C++) in which the compilation needs to performed separately for each different device platform.

6. Subroutines

“Copy and paste is a design error.” - David Parnas

In addition to the main program, a program can contain other programs as well. A subroutine is called from the main program, a method, or another subroutine in order to perform a certain task. Subroutines can receive parameters and return a value, similarly to methods. A subroutine can call another subroutine and sometimes even itself (this is called recursion). A real program consists of several subroutines, each of which performs a small task on their own. In this way, a large task can be didived into a set of smaller, easily manageable subtasks.

Subroutines are used because

• they allow dividing the program into smaller parts
• they help with structuring the program
• they are helpful for reusability
• smaller parts make testing easier

The objects in modern object-oriented languages are actually a collection of object-specific variables (attributes) and subroutines that handle them (methods). In addition, the API (Application Programming Interface) of modern languages is significantly larger than the langauge itself. In addition to subroutine libraries included in a language, also application-specific libraries, which can be very extensive, are used often. The Jypeli library used on this course is one example of an application-specific library. Using existing libraries eases the workload of programmers, because not all has to be done by themselves.

On the other hand, subroutines are also coded by the programmer himself/herself. In practice, it is often the case that the program contains a part that is often repeated almost identically. In these cases, the programmer tries to find the common factors between these parts and tranform them into a subroutine. If the functionality wasn't completely identical in repeated parts, the difference is relayed to the subroutine as parameters. This way, the same subroutine can do slightly different things with each different call. An example of this will be presented shortly.

However, sometimes subroutines are also coded when there is a problem such as "I need to find the largest number in this array". In most cases, it makes no sense to write out the search and instead the programmer makes the request: "we should have a subroutine that does this". The same in writing:

large = Largest(array);

Later on the Largest subroutine (or function in this case because it returns a value) is implemented. Now if the same task would have to be completed again, the same subroutine call could be made again (reuse).

The same subroutine is often called from a program multiple times, but for the sake of clarity it can be reasonable to code independent entities into subroutines as well (structured/top-down programming), even if they are called only once.

The following is an example of structuring, reuse, and clarifying.

If the task was to draw multiple snowmen, the solution would probably look something like this with our current know-how.

We notice that the lines of code for drawing the first and the second snowman are almost identical. In fact, the only difference is the coordinates of the snowmen. First, we try to make the lines of code for drawing both the snowmen exactly identical.

First, we could write the code so that the centre of the bottom snowball in the snowman is saved as variables x and y. With the help of these coordinates we can then calculate the position of the other snowballs. Let's also define p1, p2, and p3 as PhysicsObjects. Line numbers have been left out for clarity. At the end of this chapter, line numbering is included when we present the finished program. Remember: we can include the this self-reference when referring to the object's own attributes.

double x, y;
PhysicsObject p1, p2, p3;

// Let's make the first snowman
x = 0; y = Level.Bottom + 200;
p1 = new PhysicsObject(2*100, 2*100, Shape.Circle);
p1.X = x;
p1.Y = y;
this.Add(p1);

p2 = new PhysicsObject(2 * 50, 2 * 50, Shape.Circle);
p2.X = x;
p2.Y = y + 100 + 50; // y + radius of 1st snowball + radius of 2nd ball
this.Add(p2);

p3 = new PhysicsObject(2 * 30, 2 * 30, Shape.Circle);
p3.X = x;
p3.Y = y + 100 + 2 * 50 + 30; // y + radius of 1st ball + diameter of 2nd ball + radius of 3rd ball
this.Add(p3);

Respectively, we only need to set the correct values of x and y for the second snowman.

// Let's make the second snowman
x = 200; y = Level.Bottom + 300;
p1 = new PhysicsObject(2 * 100, 2 * 100, Shape.Circle);
p1.X = x;
p1.Y = y;
this.Add(p1);

p2 = new PhysicsObject(2 * 50, 2 * 50, Shape.Circle);
p2.X = x;
p2.Y = y + 100 + 50;
this.Add(p2);

p3 = new PhysicsObject(2 * 30, 2 * 30, Shape.Circle);
p3.X = x;
p3.Y = y + 100 + 2*50 + 30;
this.Add(p3);

Let's inspect the changes in detail.

The line above introduces two variables, the type of which are floating point numbers. A floating point number is a way of presenting real numbers in computers. In C# each variable has to have a type, and one of types of the floating point number is double. Variables and their types will be discussed in more detail in chapter 7.

The line above contains two statements. The first one sets the value of x to be 0 and the second one sets the value of yto be 50 (if, for example, Level.Bottom happens to be -150). Now we can use these variables for the calculations related to the snowballs.

Respectively, the line above sets new values for the variables which are used for calculating the positions of snowballs in the next snowman. Notice that the y-cooordinate receives a negative value, in which case the centre of the bottom snowball in the snowman descends below the middle level of the screen.

Now, the x-coordinate is set to be variable x and respectively the y-coordinate is set to be variable y, and the positions of the other snowballs are calculated based on the coordinates of the first snowball.

After these changes, the drawing process of both snowmen is performed with exactly the same code from line x= onwards.

Drawing new snowmen is now somewhat easier because all we have to do is just indicate the position of the new snowman before drawing it, and the drawing itself is simply a matter of copying and pasting the code. However, if we must copy-paste our code, we should consider if it's reasonable to do so.

In the case of drawing two snowmen, copying and pasting is still manageable without increasing the amount of code uncontrollably, but what if we have to draw 10 or 100 snowmen? How many lines of code would the program then contain? When an almost identical strip of code is appears in several places, it is usually necessary to form a subroutine of it. Pasting the same code into several places would only increase the amount of code and complicate both understanding and testing the program.

In addition, if the repeated code contained errors, corrections would also have to made in several places. One of the criteria for how good a program is is that if something needs to be changed, would these changes be done in one place only (good) or several places at a time (bad).

6.1 Subroutine calls

We want to make a subroutine that draws a snowman in a specific point. Just like methods, subroutines also receive necessary information with the help of parameters. Parameters should only convey the most minimal amount of information in order to perform the task of the subroutine.

Let's agree that our subroutine always draws a snowman of the same size in a specified point (position). What is the necessary information that the subroutine needs to draw a snowman?

The subroutine needs the information about which point the snowman should be drawn in. For this, we will give the centre point of the bottom snowball in the snowman as a parameter. The positions of the other snowballs can be calculated with the help of this centre point. Additionally, we need one parameter of the type Game so that our subroutine can be called from other programs as well. These parameters are anough to draw a snowman.

When a subroutine is used in a program, we say that we are calling a subroutine. The call can be performed by writing the name of the subroutine and giving it its parameters. The only difference between a subroutine call and a method call is that a method is always related to a certain object. For example, the ball object p1 could be removed from the game level by calling the method Destroy(); this call would be written as follows:

In other words, when we call methods, we first need to write the name of the object for which we call the method, then a full stop (.), and finally, the name of the method we are calling. The parentheses naturally contain the necessary parameters of the method. The Destroy-method above does not receive any parameters.

6.1.1 Coding subroutine calls

Let's decide that the name of the subroutine is DrawSnowman. Let's also agree that the first parameter of the subroutine is the game in which the snowman is drawn (by writing this). The second parameter is the x-coordinate of the centre point of the bottom snowball in the snowman and the third parameter is the y-coordinate of the centre point of the bottom snowball. Now we can draw a snowman with the centre of the bottom snowball at the point (0, Level.Bottom + 200) with the following call:

DrawSnowman(this, 0, Level.Bottom + 200);

The call can also start with the name of the class in which the subroutine is located. With this call the subroutine can also be called from other classes, because the access modifier of the Snowmen class is public.

Snowmen.DrawSnowman(this, 0, Level.Bottom + 200);

Although this format resembles a method call quite a lot, there is a clear difference. When we call a method, its action is always performed for a certain object, like p1.Destroy() destroys only the ball to which the object p1 refers. Of course, there can be multiple ball objects in the program (like in our example). However, the subroutine call below simply uses the DrawSnowman subroutine which is located in the class Snowmen.

If we had coded the subroutine itself already, Begin would now draw us two snowmen.

/// <summary>
/// Calls the subroutine DrawSnowman
/// with the necessary parameters.
/// </summary>
public override void Begin()
{
  Camera.ZoomToLevel();
  Level.Background.Color = Color.Black;

  DrawSnowman(this, 0, Level.Bottom + 200);
  DrawSnowman(this, 200, Level.Bottom + 300);
}

Of course, because the subroutine DrawSnowman doesn't exist yet, the program is not functional yet. In order to get the calls to work we have to implement the subroutine itself.

It is often wise to progress specifically in this order when implementing a program: first, formulate the subroutine call, write the call in the place it belongs, and only then implement the code of the subroutine itself.

More information about calling subroutines can be found in the document Calling subroutines (currently only available in Finnish):

https://tim.jyu.fi/view/kurssit/tie/ohj1/materiaali/aliohjelmienKutsuminen.

6.2 Coding subroutines

Before we start to code the functionality of the subroutine DrawSnowman, we need to introduce or declare the subroutine. Let's write the declaration of the subroutine that was already called in the previous section.

We will add the frame of the subroutine into our program. We will also document the subroutine right away.

/// <summary>
/// Calls the subroutine DrawSnowman
/// with the necessary parameters.
/// </summary>
public override void Begin()
{
  Camera.ZoomToLevel();
  Level.Background.Color = Color.Black;

  DrawSnowman(this, 0, Level.Bottom + 200);
  DrawSnowman(this, 200, Level.Bottom + 300);
}

/// <summary>
/// A subroutine that draws a
/// snowman in the specified position.
/// </summary>
/// <param name="game">The game that contains the snowman</param>
/// <param name="x">The x-coordinate of the centre point of the bottom snowball in the snowman</param>
/// <param name="y">The y-coordinate of the centre point of the bottom snowball in the snowman</param>
public static void DrawSnowman(Game game, double x, double y)
{
}

The image below clarifies the connection between the subroutine call and the subroutine declaration and its complementary parameters.

The first line of the subroutine implementation

public static void DrawSnowman(Game game, double x, double y)

is called the subroutine header or introductory line. The first part of the header (the access modifier) defines the visibility of the subroutine as public. When a subroutine is public, it can be called (or used) in other classes as well.

This subroutine is also defined static. The implementation of a static subroutine cannot use the this self-reference, because it is not associated with any object. However, its benefit is that then the subroutine can be called from any part of the program and it is not (in this case) dependent on our game; also other games can make calls to this subroutine. If we would not define this subroutine as static, it would be a method, i.e. an object function (see section 8.5.).

However, a static subroutine can also use static (global) variables and constants. Use of static variables is not recommended, but use of constants is possible.

The subroutine has the return value type void, which means that the subroutine does not return any value. As a matter of fact, a subroutine could return a value that is needed in the program when the subroutine is finished. These types of subroutines will be discussed in chapter 9.

After the definition ´void´, we named the subroutine DrawSnowman.

The parameters of the subroutine are declared within parentheses after the name of the subroutine. Before each parameter we also need to declare the data type of the parameter. In this subroutine, the provided parameters were the x- and y-coordinates of the bottom snowball. The data type of both of these is double, so the complementary parameters in the subroutine also need to be of the type double. We will also give them descriptive names x and y.

To recap, here are the words in the introductory line of the subroutine:

public static void DrawSnowman(Game game, double x, double y)

Because we decided to call this subroutine with three actual parameters in the following way:

DrawSnowman(this, 200, Level.Bottom + 300);

the introductory line of the subroutine needs to declare three formal parameters of the same type in the same order. Of course, 200 is an integer, but an integer can be assigned to a real number, so for the sake of general use, x and y are declared as real numbers in this case. Thanks to this, the subroutine can also be called as follows:

DrawSnowman(this, 10.3, 200.723);

More information about data types can be found in section 7.2. and chapter 8.

In fact, because this in the call above is of the type Snowmen, inherited from the class PhysicsGame but because PhysicsGame is inherited from the regular game class Game, variables of both the type Snowmen and PhysicsGame can be assigned to a variable of the type Game. Of course, the introductory line of the subroutine could also declare the variable type of game to be SnowMen or PhysicsGame, but then the subroutine would not be able to draw a snowman into a game of the type Game (inherited from the class Game). In other words, the generalisation here is similar to assigning 200 (an integer) into a variable of the real number type (double).

So, the subroutine call and declaration have a very strong connection. The information provided in the subroutine call (actual parameters) are "assigned" to the complementary parameters in the introductory line of the subroutine (formal parameters) with each call. In other words, broadly the following actions are performed when calling a subroutine.

the game in the subroutine = this;
x in the subroutine = 200;
y in the subroutine = Level.Bottom + 300;

Now we can run our program. It works (it runs), but of course it doesn't draw any snowmen, and it shouldn't because the subroutine we just created is "empty". Between the braces, let's add the code that is necessary to draw snowballs.

A small change to the earlier version is needed, however. Lines that add the snowballs to the field are changed as follows

game.Add(...);

in which the dots are replaced by the name of the ball object. This is because, in fact, we originally should have always written:

this.Add(p1);
this.Add(p2);
etc.

In the original snowman program we called the method Begin of the Snowman class, which was meant to declare that the snowballs need to be added to this (this) specific game (game object, which is an instance of the Snowman class). In many object-oriented languages it is possible to either include or leave out the self-reference this when referring to the object's own methods (like Add here) or attributes. Everyone can chooseone's own style, but in this material this is usually left out. Similarly, DrawSnowman is not the subroutine of a particular object (this is caused by the declaration static), which is why it receives the information about which game includes the snowman it draws as a parameter. In our example, we particularly relayed the parameter this to it. This is why the subroutine call in our example

game.Add(p1);

is specifically

Finally, the complete Begin method and DrawSnowman subroutine:

When we form the code for the subroutine functionality, we use the names we gave to parameters. The coordinates of the centre of the bottom circle are provided in the parameters x and y, but the centres of the other circles need to be calculated with the help of the coordinates of the bottom circle. This is done exactly like in the example earlier. In fact, if we compare the contents of the subroutine with the contents of the previous example, they are exactly the same.

In C#, it is customary to capitalise the names of subroutines and methods and each word within the name. This style of writing is called PascalCasing. Variables are not capitalised, but each following word within the name of the variable is capitalised: for example double speedOfCar. This style of writing is called camelCasing. More about the naming conventions of C# here:

http://msdn.microsoft.com/en-us/library/ms229043.aspx.

Let's take a look at what happens in the subroutine call.

DrawSnowman(this, 0, Level.Bottom + 200);

The call above assigns the value this (i.e. the game in question) to the variable game, the value 0 to the variable x (an integer can be assigned to a floating point number), and the value Level.Bottom + 200 to the variable y. Of course, we could also assign any other floating point number to x and y.

The execution of a subroutine call first calculates the value of each expression in the call and then these calculated values are assigned to the corresponding parameters in the subroutine in the order designated in the call. For this reason the corresponding parameters must be assigment-compatible with the expressions in the call. The example call has simple expressions: the name of a variable (this), an integer (0), and a real number (Level.Bottom + 200). However, they could also be more complex, for example like this:

DrawSnowman(this, 22.7+sin(2.4), 80.1-Math.PI);

Because we defined the type of coordinates as double, we could assign any other types of decimal numbers as parameters as well. Remember that in C#, the decimal number constant uses the dot (.) as decimal separator, i.e. to separate the integer from the decimal part.

6.2.1 Complete program

In its entirety the program looks like this:

When calling a subroutine the execution of the program jumps to the first line of the called subroutine immediately after assigning parameters and start executing the subroutine with the parameters defined in the call. When the exeution reaches the last line of the subroutine, the execution continues from the next statement after the subroutine call. In our example, when the first snowman has been drawn, the execution continues from the semicolon at the end of the first statement and then the main program continues with the call to draw the other snowman.

If we now want to draw more snowmen, each new snowman would only add one line to the code.

In C# subroutines and functions can be overloaded with different parameters. This means that the program can contain multiple subroutines with the same name as long as they have the different number (or different types) of parameters. More in section 6.5..

More information (in Finnish) about coding subroutines can be found on the course extra material page.

6.3 Documenting subroutines

According to good coding conventions each subroutine should contain a documentation comment. The documentation comment for a subroutine should contain at least the following information: a short description of the functionality of the program, a description of all the parameters, and an explanation of the return value. This information is described in tags in the following way:

• The documentation comment starts with the summary tags; the text between the tags is a short and clear description of the functionality of the subroutine
• Each parameter is described between the param tags, and
• Each return value between the returns tags

The documentation comments for the DrawSnowman subroutine are on lines 36-42 in our previous example.

36 /// <summary>
37 /// A subroutine that draws a
38 /// snowman in the specified position.
39 /// </summary>
40 /// <param name="game">The game that contains the snowman</param>
41 /// <param name="x">The x-coordinate of the centre point of the bottom snowball in the snowman</param>
42 /// <param name="y">The y-coordinate of the centre point of the bottom snowball in the snowman</param>

You can try documentation in the previous complete Snowman example by clicking the Document link. Then try to different links in the documentation to see what's behind them. The same can be seen in the image below.

The HTML page produced from this class by the Doxygen tool (see: http://en.wikipedia.org/wiki/Doxygen) would look like this:

The documentation shows all the subroutines and methods in the class. Notice that Doxygen names both subroutines and methods as member functions. As said, naming conventions vary in literature, and in this case the naming convention resembles the one used in C++. However, member functions are called subroutines and methods on this course.

The details of each subroutine and method can be found in the section Detailed Description. The documentation of the DrawSnowman subroutine with its parameters can be seen at the bottom of the image.

6.3.1 Note

All of the information needed in the DrawSnowman subroutine was relayed with the help of the parameters and no external information was needed during the execution of the subroutine. This is typical and usually also a desirable feature of subroutines. In these cases, the subroutine is defined as static.

6.4 Subroutines, methods, and functions

As you may have noted, subroutines and methods have a lot in common. Many sources also call subroutines methods. In these cases subroutines are separated from object methods by calling them static methods. However, in this material, methods refer to object functions only. In Jypeli documentation, we can study static methods of the RandomGen class which generate random number for example. A single ball was removed with the method Destroy, which is an object function.

Ww talk about subroutines on this course because the term is used in many other programming languages as well. This course is primarily a programming course that uses C#. Our main goal is therefore to learn how to program and we use the C# language as a tool for learning it.

Our subroutine DrawSnowman did not return any value (void). Subroutines (or methods) that return a value can also be called functions more specifically.

Different names are used for subroutines and methods in different languages. For example, in the C++ language, both subroutines and methods are called functions. In C++ methods are more specifically called member functions, like Doxygen did in the case of C# as well.

Let's revise the differences of subroutines, functions, and methods in brief.

Subroutine: a general name for any kind of subroutine, function, or method. A word subroutine does not specicy the number of parameters or return value. In void-type subroutine there can be a return-sentence, but without a value. Then the only meaning for return is to jump out of from the subroutine.

In some languages, for example C++, all different types of subroutines are called functions. In Java literature all subroutines are usually called methods.

On this course, the gereral name subroutine is used when we don't want to emphasise that the block is especially a function or a method. Next, let's specify these concepts.

Function: a subroutine that returns a result value, for example the average of two numbers. According to this definition, a function always contains at least one return statement followed by an expression, for example return (a+b)/2.0; Even a void subroutine (i.e., a subroutine that doesn't return a value) can contain a return statement, but it can't be followed by an expression. A function should usually be static.

Method: a subroutine that uses the information of an object for performing a task. Methods are used on this course (for example string.IndexOf), but not coded except for methods of the game class (for example Begin). At the end of the course, some can also potentially make a new class and then write own methods for it. In practice, a method uses the this reference and for this reason it cannot be static.

A method can also return a value, similarly to a function, or it can be void, i.e. not return a value, similarly to a subroutine.

6.4.1 Coding subroutines

When coding subroutines, it would be best to progress like this (as long as we learn how to test, TDD, Test Driven Development):

1. Separate the problem into parts.
2. Come up with a descriptive subroutine call that starts the solving of a certain problem.
3. Write the call line of the subroutine and think what parameters it requires.
4. Write (first manually, later generate automatically) the introductory line (the header) of the subroutine.
   • think about if it needs the words public,static
   • is the return type of the subroutine void or something else?
   • the name of the subroutine
   • the same number of parameters as in the subroutine call.
   • assignment-compatible types of parameters to the call.
5. Make a syntactically correct stub of the subroutine that compiles, for example a function subroutine needs to a have a return statement that return an expression (for example one number) that is of the same type (or is automatically turned into the same) as the type of the function.
6. Document the subroutine (don't mention where it was called, it is not important here)
7. Write the tests (TDD) - see the note about testing below
8. Run the tests (they should fail == you will see a red bar).
9. Make the subroutine functional.
10. Run the tests (repeat phases 8-10 until it works = you will see a green bar).
11. Move onto the next subroutine.

Read more in the document Writing subroutines (in Finnish).

The instructions above are the common "recipe" for coding subroutines. It includes tests, but with the information provided on this course we can only test the functions that are discussed later in this material in chapter Return value of a subroutine. In other words, the "recipe" above cannot be used properly before we learn more about functions and testing. Subroutines that print out something cannot be tested with the information from this course.

Of course, subroutines like the ones above are not very efficient and it would be more sensible to give the subroutine the text it should print as a parameter.

6.4.2 Task: Terminology

/// <summary>
/// Calls the DrawSnowman subroutine
/// with necessary parameters.
/// </summary>
public override void Begin()
{
    Camera.ZoomToLevel();
    Level.Background.Color = Color.Black;

    DrawSnowman(this, 0, Level.Bottom + 200);
    DrawSnowman(this, 200, Level.Bottom + 300);
}

6.5 Overloading subroutines

In C# subroutines and functions can be overloaded with parameters. This means that a program can contain multiple subroutines with the same name that have a different number or types of parameters. This can be utilised so that the function that takes in more parameters can do more or do the same operation with more details than a function that takes in less parameters.

6.5.1 A simple example

First, let's take as simple an example as possible about overloading. The function in case sums numbers.

First, we make a function that returns the sum of two numbers.

public static double Sum(double a, double b)
{
  return a + b;
}

This can be called from the Main program by writing

double sum = Sum(5, 10.5);

Then we say hey, we also need a function that can sum three numbers, and we want to call it by writing

We write a function with the same name but in the introductory line (function signature) we give three parameters instead of two. We will implement the function immediately as well.

But now we notice that we have almost similar code in two function. We will change the first function so that it calls the first function (that can do less) calls the second function (that can do more). We will provide 0 as the third number (third parameter).

public static double Sum(double a, double b)
{
  return Sum(a, b, 0);
}

With this example we learned what overloading means. The next example brings out the benefits of overloading better.

6.5.2 Standard-size snowman vs. size of snowman as parameters

We can create a standard-size snowman with the following subroutine.

We can call the subroutine in Begin like this for example.

But what if we want to draw snowmen of different size sometimes? In other words, it would be enough if the DrawSnowman subroutine drew a snowman of different size in addition to a standard size snowman. The calls in Begin could look like this.

But now Visual Studio shows the error

No overload for method 'PiirraLumiukko' takes 4 arguments.

So now we will write a new subroutine called DrawSnowman (yes, the same name) but the in addition to the game and position parameters we also provide the radius of the bottom snowball.

Move the code from the original subroutine to this new subroutine and set the radius of snowballs to be dependent on the radius provided as a parameter. Additionally, set the positions of the middle and top snowball to be dependent on the size of the snowballs! The new (four-parameter) subroutine would look like this.

We can now call this "version" that does more from the three-parameter DrawSnowman subroutine, which saves us from copy-pasting code.

.

7. Variables

Variables act as data storage for different things in programs. A variable is like a small box that can store things like numbers, words, information about the user of the program, and much, much more. In procedural languages, processing information would be impossible without variables. In functional programming, things are a little different. We have already used variables, for example in the Snowman example we created the PhysicsObject-type variables p1, p2, and p3. Accordingly, the parameters (Game peli, double x, double y) in the DrawSnowman subroutine are also variables: the Game-type object variable game and the double primary-type variables x and y.

The term variable is borrowed from mathematics, but these two concepts should not be confused with each other - a variable in mathematics and a variable in programming have a slightly different meaning. You will notice this in the next sections.

The values of variables are saved in main memory or registers, but in programming languages we can give each variable a name (an identifier) in order to make it easier to handle them. The naming of variables makes programming easier, because then the programmer does not need to know the addresses of the information they need in the main memory or registers; it is enough to remember the name of the variable the programmer himself/herself named.

Because the compiler needs to reserve a memory area of the right size for the variable, the type of the variable also needs to be introduced. The type of the variable is also needed in order to know how to handle the information saved in the memory location. In order to understand the different ways in which data types are saved, we will get familiar with binary numbers etc. later. For example, the combination of eight bits, aka the byte 01000001can be interpreted as the letter A or as the natural number 65 etc.

For example, the statement Console.WriteLine(a) would not know what to print unless the type of the variable a was known. Accordingly, if you hear the word knight, you would not tell it apart from the word night unless you are given the context as well.

7.1 Defining variables

When a mathematician says that "n is equal to 1", it means that the term (or variable) n is in some inexplicable way equal to the number 1. In mathematics, variables can be introduced as sporadically as this.

However, a programmer needs to be more specific about variables. In C#, values are assigned to variables like this:

The first line roughly translates to "chip a small piece - the size of an ìnt value - of storage space from the memory of the computer, and use the name n for it from now on". The second line declares that "save the value 1 to the variable, which has the name n, in this way replacing whatever may already be in the storage space".

The character = is the assignment variable, which will be discussed later.

Then what is the int in the previous example?

In C# each variable must have a data type (usually type in short). The data type must be defined so that the program would know what kind of information is saved into the variable. On the other hand, the data type also needs to be defined so that the program knows how much space to reserve from the memory for the information in the variable. For example, in the case of an int variable the required space is 32 bits (4 bytes), the byte variable requires 8 bits (1 byte), and a double variable requires 64 bits (8 bytes). A variable is declared by writing the data type, followed by the name of the variable. Variable names are not capitalised in C#, but each word in the name that follows is capitalised. As was mentioned before, this naming convention is called camelCasing.

variableDataType variableName;

The int we mentioned is a data type, which stores integers. We can assign the numbers 1, 2, 3, and also 0, -1, -2 etc. in the n variable , but not the number 0.1 or the word "Hey". However, whatever we assign to it, the variable can only have one value at a time. If a variable is assigned a new value, the previous value can no longer be accessed in any way.

A person's age could be saved into the following variable:

Notice that we are not assigning any value to the variable, we are simply defining the variable type to be int and naming it.

Multiple variables of the same type can be defined at once by separating the names of variables with commas.

The data type double is used when we want to store decimal numbers.

Definition can also be done separately (which is even more preferable):

Variables can also be assigned values already when defining them. Then we talk about initializing variable. Note that the value can also be the result of an expression.

variableDataTypemuuttujanTietotyyppi variableName = CONSTANT;
variableDataType variableName = expressionThatProducesValue;

The values (or values of expressions) assigned to variables should be of the type that can be assigned to the variable in question. For example, an int variable cannot be assigned the following real number:

but a real number can be assigned the integer

Notice that the decimal separator in real numbers (such as double) is always a dot (.) and thousands are not separated.

7.2 Primitive data types

The data types in C# can be categorised into primitive data types (aka basic types, basic data types) and reference types. Reference types include for example the PhysicsObject type that we used earlier, like p1 etc. and the string object for saving character strings. Reference types will be discussed in more detail in chapter 8.

Different data types require a different amount of capacity in the computer's memory. Even though computers nowadays have a lot of memory, it is extremely important to select the right type of variable for each situation. In large programs this problem is magnified very fast if we use variables that use up too much memory in relation to the operation they perform. The primitive data types in C# are listed below.

Table 1: The primitive data types in C# in size order.

During this course, the most important primitive data types are bool, char, int, and double.

In this material, it is recommended that the data type used for saving decimal numbers is always double (in some cases even decimal) even though many other sources use float as well. This is because floating point numbers, which are used in computers for handling decimal numbers, are rarely accurate values in computers. In fact, they are only accurate when they represent the combinations of any two powers, like 2.0, 7.0, 0.5, or 0.375, for example.

Most often, floating point numbers are only approximations of real numbers. For example, the number 0.1 cannot be presented accurately with bits in primitive data types. In this way, the inaccuracy only grows as the number of calculations rises. For this reason, it is always safest to use the double type, because thanks to its larger number of bits it can store more significant decimals.

In certain applications where high accuracy is necessary (for example in banking and nanophysics applications), it is recommendable to use the type with the highest possible accuracy, the decimal type. The presentation of real numbers in computers will be discussed in more detail in section 26.6.

The next example demostrates what happens when we sum two too large integers or increment the variable too much in other ways.

7.3 Assigning values to variables

You can assign values to variables with the assignment operator =. Statements in which variables are assigned values are called assignment statements. It is important to notice that an assignment always happens from right to left: the value that is assigned is on the right side of the equal sign and the target variable is on the left side of the sign.

Note that a decimal point is used in real number constant, not a comma.

The variable needs to be defined as a certain type before it can be assinged a value. Variables can be assigned only values of the defined data type or assignment compatible values. For example, floating point number types (float or double) can also be assigned integer values, because integers are a subset of real numbers. In the example below, we assign the value 4 to the variable number2 and on the third line we assign the value of number2 (4) to a variable named number1.

This cannot be done vice versa: a value of the type double cannot be assigned to an int variable. The code below could not be compiled:

If the int <- double assignment above would be absolutely necessary to make, we would have to use a variable transformation aka a typecast (try in the example above, change also 4.0 to 4.8. To start with, however, typecasting is always a bad solution.

   number1 = (int)number2;  // force the number2 to be of the type int. The number is rounded down.

When a decimal variable is initialized with a number, the number needs to be followed (before the semicolon) by m (or M). Accordingly, initialising a float variable needs to be followed by f (or F).

Notie that a char variable is assigned a value by writing it within apostrophes like this:

This sets it apart from assigning values to the string variable (discussed in detail later), which is done by placing the string within quotation marks, like this:

An assignment statement can also contain more complex expressions, for example arithmetic operations:

An assignment statement can also contain variables.

So the assigned value can be any expression that produces a value that is compatible with the variable type. By combining variable and operations, an expression can be even more "complex" than the previous examples:

C# requires assigning a value to the variable before using it. The compiler won't compile a code in which a variable with no assigned value is used. The program below would not be compiled.

The error message looks ike this:

Example.cs(7,34): error CS0165: Use of unassigned local variable 'age'

The compiler tells that there is an attempt to use the age variable before any value is assigned to it. This is not allowed, so the attempt to compile the program ends here.

7.3.1 The target of assignment is always on the left

The variable for which the value is assigned is alawys on the left side of the statement. The right side of the = assignment operator is always some expression, the value of which is always calculated before the assignment and this value is assigned to the variable.

7.3.2 Task 7.4 assigning the value of a to b

First, answer the multiple choice questions below and then complete the task after it.

7.3.3 The value of the variable changes when a value is assigned to it

The value of a variable only changes when a value is assigned to it. Primitive variables are always assigned a value. If the value of a another variable is assigned to a variable, the variable receives the value which the other value has at the moment of assignment. In the following example, try changing the value of i and observe that it no longer affects the value of the sum variable:

7.3.3.1 Task 7.5 Incrementing i, what the program prints

7.4 Naming variables

The names of variables must describe the information saved in them. Usually a letter alone is a bad name for a variable, because it rarely describes the variable very well. Descriptive variable names clarify the code and reduce the necessity to write comments. A short variable name has no intrinsic value; only two decades ago it may have had because it sped up the coding process. However, with modern development environments this no longer holds true, because editors can fulfill the variable names while writing the code, so in practice the names of variables never have to written in full after defining them.

Single letter variable names can still be used when justified, if for example they have a meaning in mathematics or physics. The names x and y are good variable names for coordinates. The name l refers to length and r to radius. In a physics program the name s can be used for distance.

According to the coding conventions of C#, the name of a variable begins with a lowercase letter. If the name of the variable consists of multiple words, each new word in the name is capitalised, like in the example below.

int bicycleTyreSize;

In C# the name of a variable can contain scandinavian letters, but their use is not recommended, because then moving from one character encoding to another will usually cause extra problems.

7.4.1 C# keywords

Names of variables cannot be any of the reserved words in the used programming language, i.e. words that have another meaning and function in C#.

Table 2: C# keywords i.e. the "reserved words".

7.5 The scope of variables

The scope of variables refers to the usability of variables in different situations. If the variable is "in scope", the variable can be used in this particular point in the code.

A variable can only be used (read and assigned values) in the block where it has been defined. A block starts with the brace { and ends with the brace }.

 {
    int number = 5; 
 }

Variables exist for as long as the block is not exited. During a subroutine call the block has not been exited, because the execution returns to the block after the subroutine has been executed. A nested block does not cause exiting either.

 {
    int number = 5; 
    SubroutineCall(); 
    {
        number++; 
    }
 }

The definition of a variable must always precede (be earlier in the code) its first use. Even within the same block the variable needs to be defined before its use, because the variable turns visible only after it has been defined.

In the following example, number and d are in scope and transformable only in the main program (except if taken as an out-parameter in C#). Every defined variable in the main program (or in any other subroutine) live until the closing brace } of the main program. Here the value of the variable is copied into a corresponding parameter in a subroutine. The subroutine cannot "see" the variable in the main program in any way, rather the subroutine only receives the value from the information that has been relayed to it.

The variable defined in the subroutine cannot be viewed by other subroutines; this is called a local variable. Variables number and d are local variables in the Main program.

In the example the name of the parameter variable is the same number as in the main program, but the name could be anything else. The most important point is that in the subroutine call, the variable in the corresponding place in the subroutine is assigned the same value as in the program that calls it. Even if the variable number was assigned some value, it won't affect the program that calls the subroutine, because number is its own local variable in the subroutine and only exists until the execution of the subroutine reaches the closing brace } of the subroutine.

When compiling, the program warns that the subroutine variable newValue is not used after it has been assigned a value. If the subroutine was called again, a new newValue variable would be created with the call, and it wouldn't have anything to do with the corresponding value in the previous call.

The auxiliary variable newValue is in scope after it has been defined in the subroutine, but it stops existing after the closing brace }. Changes made to this variable (even if there is a variable of the same name somewhere) in no way affect any other place than this variable. The main program or anything else in the program cannot reach this variable in any way (except in this case this value depends on the number variable given as a parameter)

Changing parameter variable is not usually considered as abiding good conventions. If the parameter variables need to be changed, it's better to make a local copy of it and change the local copy so that by the end of the subroutine, the parameter still has the same value as it did at the beginning of the subroutine.

Below, we have defined subroutines within a class. All variables are local variables.

A variable can also be defined to be in scope everywhere within a class, so for all subroutines. When a variable is in scope to all parts in a program, it is called a global variable. Global variables should be avoided whenever possible.

The variables points and result above are global variables, because they are defined outside of subroutines. They are also in use from classes outside this class, because they are unfortunately defined with the access modifier public. If the word static was missing from the definition of the variable, they could not be used in static subroutines. In this case, the variables would be attributes and to use them we would have to create an object which would contain and define the attributes. This is extra information that is not a part of the course's main content.

Next, try what happens if you write the assignment d=4 in the subroutine Change.

However, C# does not allow a nested block use the same variable name that has been used in the outer block. However, if a global and a local variable have the same name, the local variable is in scope in its own block.

Try to comment the line i=9 away so that the program is compiled and prints the local i in the main program. If the line i=5 is commented away too, the global variable i is printed.

More information (currently only in Finnish) about the scope of variables can be found on the extra material page of the course.

7.6 Constants

One man's constant is another man's variable. -Alan Perlis

In addition to variables, constants can also be defined in programmming languages. The values of constants cannot be changed after definition. In C# a constant is defined like any other variable, but the additional modifier const is provided before the type of the variable.

In this course, constants are uppercase and words within the name are separated with an underscore (_). This way they are easily distinguishable from variable names that are not capitalised. Other ways to write constants exist, for example Pascal Casing is the second most popular convention for writing constants.

7.7 Operators

We often need to save the results of different calculations into variables. In C# calculations can be done with arithmetic operations, which were already mentioned in the context of the snowman example. The arithmetic calculations in programs are called arithmetic expressions.

C# also contains comparison operators, logical operators, bitwise operators, shortcut operators, the assignment operator =, the is operator, and the condition operator ?. The most important operators will be discussed in this chapter.

7.7.1 Arithmetic operations

Basic calculations in C# are performed with arithmetic operaions, of which + and - were already handled in examples before. There are five arithmetic operators.

Table 3: Arithmetic operations.

Note: 18/7 = 2 4/7. The integer division / returns 2 and the remainder returns 4. Remainders are often used to test if a number is divisible by some number, for example:

7.7.2 Comparison operators

Comparison operators compare the values of variables with each other. Comparison operators return a truth value (true or false). There are six comparison operators. More comparison operators are presented in chapter 13.

7.7.3 Shortcut operators

Shortcut operators allow presenting calculations in a more concise format: for example ++x; (four characters) means the same as x = x+1; (six characters). They can also be used to initialize variables.

Table 4: Shortcut operators.

The increment operator (++) and the subtraction operator (--) can be used before of after the variable. When used in front of the variable, the value is changed first and the possible action, for example printing, is performed afterwards. If the variable is followed by the operator, the function is performed first and the value is changed afterwards.

7.7.4 The order of arithmetic operations

The order of arithmetic operations is equivalent to the order of operations in mathematics. Multiplication and division is always performed before addition and subtraction. Also, the expressions in parantheses are performed first.

7.8 Remarks

7.8.1 Assigning integers to floating point number variables

When we attempt to save the result of a division of integers into a floating point number type (float or double) variable, the result may be saved as an integer if the divisor and the dividend are both entered without the decimal part.

However, if even one of the numbers in the division provided in decimal format, the result of the operation is saved into the variable correctly.

When making calculations with floating point numbers, it is best to keep all numbers in decimal format, even the integers, for example, by giving the number 5 in format 5.0.

When making calculations with integers, note the following:

As we can see above, integers are not rounded to the nearest whole number, but rather the decimal part in division is "lost" in C#. If both the divisor and the dividend are integer variables, the division is cut into an integer. This problem can be avoided by starting off with a real number term.

7.8.1.1 Task 7.8

See all answer options below. First, think about the answer for each question and only then view the lecture video to see the right answer.

7.8.1.1.1 What is the result of the following expressions?

7.8.1.1.2 What are the values of the variables?

7.8.2 About addition and subtraction operators

There are four ways to increment the value of a number by one.

In programming, an idiom refers to the way in which something should be done. From the example above, a++ is an established practice in programming and the most recommended option, i.e. an idiom. However, if the variable a should be incremented (or subtracted) by two or three, this practice would not work. The next example illustrates different ways to subtract by two. There are three alternatives for this.

In this example, the use of the += operator is the most recommended option, because the number to add can be positive or negative (or zero), so the += operator does not limit what kinds of numbers can be added to the variable a.

7.8.3 Be careful not to divide by zero

One of the most common programming mistakes is dividing by zero. This is not a syntax error, because it can seldom be noticed during compilation. In other words, dividing by zero is a logical error that goes unnoticed until during runtime. The programmer needs to make sure that the divisor is never zero. However, the conditional statement (if), which will be introduced later, is needed for aid here:

7.8.4 Numeeristen tietotyyppien arvo-alueet

Numeeristen tietotyypin pienin ja suurin mahdollinen arvo saadaan

tietotyyppi.MinValue
tietotyyppi.MaxValue

Reaalilukutyypeille on myös

tietotyyppi.Epsilon

joka kertoo pienimmän positiivisen arvon jonka muuttuja voi saada. Tästä seuraava pienempi arvo on 0.

Näitä voidaan käyttää hyväksi esimerkiksi siten, että kun etsitään vaikkapa kokonaislukutaulukon suurinta lukua, laitetaan ehdokas funktion aluksi pienempään mahdolliseen arvoonsa, jolloin kuka tahansa "voittaa sen:

        int ehdokas = int.MinValue;

7.9 Example: Body Mass Index

Let's make a program that calculates a person's body mass index (BMI). BMI is calculated by dividing the weight (kg) by the height (m) squared, i.e. with the equation:

weight / (height * height)

in C#, the BMI can be calculated as follows.

Jypeli class list:
http://kurssit.it.jyu.fi/npo/material/latest/documentation/html/classes.html

8. Object data types

The primitive data types in C# provide a very limited framework for programming. They can only store numbers (int, double, etc.), single characters (char), and truth values (bool). Even slightly more complex programs require more advanced structures for storing information. In C#, Java, and other object-oriented languages these structures are provided by objects. In C#, even a string of characters (string) is an object.

8.1 What are objects?

An object is a data structure that is aimed to represent a real world phenomenon in programming. In class-based languages (such a C#, Java, and C++), the structure and behaviour of an object is defined by its class, which describes the attributes and methods of the objects created from it. Objects have different attributes, and methods describe the functions of an object. An object is said to be an instance of the class. So, multiple objects with the same attributes and functions can (usually) be created from one class. Attribute values form the state of the object. Note however that even if several objects have the same state, they might still have a different identity. For example, two exactly identical circles can appear in the same position (=look like one circle), but in reality there are two different circles.

Objects can be created on your own, or you can use ready-made objects in libraries. Making your own object classes is not required on the Programming 1 course, but using object classes is. Next, we will study the relationship between a class and an object, and how to use an object.

The relationship between a class and an object can be described with the following example. Let's say that there are several people in a lecture hall. Everyone in the lecture hall is a person. They have certain features that all people have, for example a head, a nose, and other body parts. However, everyone in the lecture hall is a different instance of a person, so each object has their own identity - they are not one and the same, there are several of them. Different people can have different hair and different colour eyes and their own manner of speech. Additionally, people can be of different height, weight, etc. Even identical twins in the lecture hall would each be a different instance of a person, i.e. a Person object. Hair, eyes, height, weight would be object attributes. A Person might also have functions, i.e. methods, for example Eat, GoToWork, Study, etc. Next, we will study a more conventional example of objects.

Let's assume that we are designing a pay system for a company. We would need an Employee class, among others. The Employee class would need at least the following attributes: name, job, department, salary. The class would have at least the following methods: PaySalary, ChangeJob, ChangeDepartment, ChangeSalary. Each employee would be their own instance of the Employee class i.e., an object.

8.2 Creating objects

Employee joe = new Employee("Joe Bungler", "Project manager", "Research department", 5000);

Object references are defined by first writing the name of the class from which the object is created. Next, we name the object ("joe" in the example above). After the name, we write the equals sign, after which we write the word new to inform that we are creating a new object. This new operator reserves space for the object from the memory of the computer.

Next, we write the name of the class again, after which we write (in parentheses) the parameters that are possibly required to create the object. Parameters depend on how the class constructor has been implemented. The constructor is a method that is always executed when a new object is created. However, in order to use ready-made classes, you don't have to know about the implementation of the constructor, because the requires parameters can be read from the class documentation. In general format, a new object can be created in the manner below.

Class objectName = new Class(parameter1, parameter2,..., parameterN);

If an object doesn't require any parameters for creating it, an empty pair of parentheses is written.

A new Employee object could be created for example in the following way. The parameters depend on how the constructor of the Employee class has been implemented. In this case, we provide all the attributes as parameters to the object.

Employee donaldDuck = new Employee("Donald Duck", "Manager", "Department3", 3000);

At the beginning of this material, we drew snowmen by creating objects of the PhysicsObject class as follows.

In fact, an object variable in C# is only a reference to an object. This is why they are often called reference variables as well. Reference variables differ significantly from primitive data type variables.

8.3 The difference between object data types and primitive data types

C# contains two types of structures that can store information. Depending on the case, information is either stored into primitive data types or object data types. Object data types differ from primitive data types in that they are references to a certain object and, for this reason, they are also called reference types or reference variables.

• Primitive data types store their information in one place in the computer memory (a stack)

• Reference types contain a reference to another place in memory (a heap), where the data itself is located. However, the reference to the object remains in the stack.

Muuttujien luominen ohjelmassa vaatii muistitilaa tietokoneen keskusmuistista. C# varaa muistista tilaa muuttujan sisältämälle tiedolle (yllä olevassa esimerkissä 3 ja { 1, 2, 3 }) jommasta kummasta kahdesta muistialueesta: pino tai keko. Tällä kurssilla pääsääntö on seuraava: arvopohjaisten tietotyyppien data sijaitsee pinossa ja viitetyyppien data sijaitsee keossa.

Tarkasti ottaen arvopohjaisten muuttujien arvot voivat sijaita joko pinossa tai keossa riippuen siitä, missä kontekstissa muuttuja on määritelty. Esimerkiksi Henkilö-luokka (viitepohjainen, sijaitsee keossa) voisi sisältää int-tyyppisen ikä-attribuutin. Tässä tilanteessa myös ikä sijaitsisi keossa, ei pinossa.

Usually we won't have to worry about whether we use primitive data types (like int, double, or char) or object data types (like string). Generally, the most important difference is that primitive data types should (apart from a few exceptions) always have a value, but object data types can also have a null value (i.e. "nothing" value). Later, some examples of the differences between primitive data types and reference data types will be presented.

Several variables can refer to the same object. Compare the lines of code below.

The code above prints "10" as it should. The value of variable number2 won't change even when we assign the value 0 to number1 on line three. The reason for this is that on the second line, we assign the value of number1 to the variable number2, and not the reference to variable number1. Object data type variables act differently. Compare the example above to the following example:

The code above draws the following image:

We could make the rash assumption that the image just shows two similar circles in the same location. This is not the case, however: both of the PhysicsObjects refer to the same circle with radius 50. This is caused by the fact that variables p1 and p2 are object references which refer (or point) to the same object.

PhysicsObject p2 = p1;

In other words, the line above does not create a new PhysicsObject, only a new object reference which now refers to the same object as p1.

References are discussed in detail in chapter 14.

8.4 Calling methods

Each object created from a certain class has all the public methods of the class in its use. In the method call we command the object to do something. For example, we could command the PhysicsObject to move, or the Employee object to change its salary.

Object methods are called by writing the object's name, a dot (.), and the name of the method to call. The possible method parameters are placed within parentheses and divided by commas. If the method doesn't require any parameters, the parentheses are still required, they simply won't contain anything. The method call in general format:

nameOfObject.MethodName(parameter1,parameter2,...parameterN);

For example, we could change the salary of the donaldDuck object with the following.

donaldDuck.ChangeSalary(3500);

Or make the p1 object (assuming that p1 is a PhysicsObject) move with the Hit method.

String class has for example the Contains method that returns either the value True or False. The Contains method is given a string as a parameter, and the method will search for instances of the provided string in the object. If the object contains the string (once or several times), it returns True. Otherwise it returns False. An example of this below.

8.5 The difference between methods and subroutines

A subroutine is introduced as static if the subroutine does not use any other information apart from the information provided as parameters. For example, section 20.4.2 contains the following subroutine.

private void ListenToMovement(AnalogState mouseState)
{
   ball.X = Mouse.PositionOnWorld.X;
   ball.Y = Mouse.PositionOnWorld.Y;

   Vector mouseMovement = mouseState.MouseMovement;
}

Here, in addition to the mouse state, the information of the ball object introduced in the game object (this) is required, so this is no longer a static subroutine, which is why the word static was left out. However, a method can use the object's own attributes, methods, and the so-called property fields. Remember that the object's own properties can be referred to like this as well:

   this.ball.X = Mouse.PositionOnWorld.X;

so if the subroutine needs the this reference, it is a method (not static).

8.6 Destroying objects and garbage collection

When there are no variables (object references) that refer to an object anymore, the memory locations must be freed for other use. The objects are removed from memory with the help of a cleaning operation. In C#, garbage collection takes care of this. When there are no longer any references to an object, it is marked as removable, and every now and then the garbage collector frees the memory locations of the marked objects.

All programming languages don't have this feature (e.g. original C++), in which case freeing up memory and destroying objects needs to be taken care of manually. These languages usually have a destructor which is executed whenever an object is destroyed. Self-made destructors usually call the destroying of the objects created during the object's life cycle and freeing of other resources. Compare to the constructor which is executed when the object is created. The challenge in these languages is that sometimes the life cycle of objects is automatic in some cases and sometimes not. This can easily cause a memory leak, i.e. a memory location is not freed up, but it no longer has any pointers to it that would enable using it, which will leave the memory location reserved for the remainder of the program. Memory leaks are very common for starting C++ programmers. Languages like Java and C# have made it remarkably easy to avoid memory leaks.

Usually C# programmer don't have to worry about freeing up memory, but there are certain situations in which you may have to remove objects by yourself. One example of this is handling files: if an object has opened a file, it would be sensible to close the file before destroying the object. In this case, closing the file would have to be performed in the same context as destroying the object. This could be done by introducing a destructor which is a class method that strips the object of all the information it contains and frees up all the constructions it contains, like links to open resources (e.g. files; however, it is not recommended to keep files open for times as long as the duration of the entire life cycle of an object)

8.7 Documentation of object classes

Class documentation contains information about the class, the constructors of the class, and methods. Class documentation usually contains links to examples, like in the case of the String class. Now, we will study the documentation of the String class in more detail. The documentation of the String class can be found here: http://msdn.microsoft.com/en-us/library/system.string.aspx. The documentation contains e.g. a list of members, i.e. the constructors, attributes (fields), properties, and methods that can be used.

At this point, we are interested in the String Constructor and String methods (in the hierarchy tree on the left side of the page). Click String Constructor to get more information about the class constructors and click String Methods to get more information about the methods that can be used.

8.7.1 Constructors

Open the String Constructor page of the String class. This page contains all the information about the class constructors. There can be several constructors as long as their parameters differ in some way. Each constructor has its own page, and each programming language has its own version (the .NET Framework contains several programming languages). Naturally, we are now interested only in the C# versions.

For each constructor, there is a short description of what it does, which types of and how many parameters it receives. You can click the introductory line of the constructor to see more information about it. For example the link

[C#] public String(char[]);

takes you to the page (http://msdn.microsoft.com/en-us/library/ttyxaek9.aspx) which contains more information and usage examples of the constructor public String(char[]).

Note that many of the constructors in the String class have been marked as unsafe, which means that they should not be used in your code. These types of constructors are only used for inter-system communication.

At this point it might be difficult to understand the meaning of all the constructors, because they contain data types that we haven't discussed yet. For example, the square brackets after the data type (e.g. int[]) mean that the data type is an array (a table). Arrays will be discussed in chapter 15.

A String object is perhaps the most common object in C#, and it is in fact a collection (an array) of consecutive char type characters. It can be created as follows.

Of course, this way of writing strings is burdensome. However, a String object can exceptionally be created in a manner that resembles the definition of primitive data types. The statement below is equivalent to the example above, but it is much shorter.

Note that the string is written with quotation marks. The keyboard shortcut is Shift+2. In a similar manner, a string could be initialized with any other String class constructor, of which there are many.

If we study the documentation of the PhysicsObject class (found here: http://kurssit.it.jyu.fi/npo/material/latest/documentation/html/ -> Luokat -> Luokkalista -> Jypeli -> PhysicsObject), we find multiple different constructors (see the list of public member functions, Julkiset jäsenfunktiot, and find the items that begin with the word PhysicsObject). The second one on the constructor list takes in two numbers and a shape as parameters. This constructor was already used in the snowman example.

However, we could leave out the shape (the first constructor) and define the shape later with the Shape attribute of the PhysicsObject class.

8.7.2 Exercise

Study the other constructors. What can you find out based on the documentation? What is the default shape?

8.7.3 Methods

The section Methods (http://msdn.microsoft.com/en-us/library/system.string_methods.aspx) contains information about all the String class methods. Each method has its own line in the table and each line has a short description of what the method does. When you click on a method, a page with information about it is opened. This page contains information about e.g. the parameter types the method takes in and its return value type. For example, the String class contains the ToUpper method which returns a String type value.

8.7.4 Note: Looking up class documentation

Note that when you search for class documentation, the search engine results can refer to the older versions of the .NET Framework (for example 1.0 or 2.0). At the moment of writing this, the latest version of the .NET Framework is 4 and you need to be make sure that the documentation you find refers to the right version. You can use the version number in your search like this: "c# string documentation .net 4". The version number is located under the title of the document. You can change between versions by clicking the drop-down menu Other Versions.

8.8 Typecasting

In C#, a variable can only store one type of information. For this reason, we sometimes need to change a String varibale into an int variable or a double variable into an int variable etc. Changing a variable type into another type is called typecasting.

All primitive data types and C# object types have the ToString method, which transforms the object into a string. Below is an example of typecasting an int to a string.

Changing a string to a primitive data type can be done with the help of the equivalent class method for each primitive data type. As we can remember, primitive data types are not objects, so they do not have methods. However, each primitive data type has an equivalent struct in C#, which contains useful methods for handling primitive data types. Structs are located in the System namespace, which is why the following statement is needed at the beginning of the program

Structs equivalent to primitive data types are in the following table.

Table 5: Primitive data types and their equivalent structs.

Note that the struct and data type names are synonymous in C#. The following lines produce the same result (if the System namespace has been utilised with the using statement).

All the struct methods are in use, regardless of if you write the name of the primitive data type or the name of the struct. Here is an example.

In C#, a String type can be changed into an int with the int.Parse function as follows.

Specifically, the Parse function creates a new int based on the string it received as a parameter, which is stored into the variable number2.

If parsing a number fails, this causes a so-called exception. Parsing a double can be done with the Parse function found in the Double struct (note the capital D).

In practice, if the information is entered by a person, it is very likely that it doesn't form a legitimate number. For this reason it is often more useful to use the function TryParse:

Things are made even more complex by the fact that the operating system may use either the comma (,) or the dot (.) as the decimal separator.

9. The return value of the subroutine

The Snowman subroutine we created in the Subroutines chapter didn't return any value. However, it is often useful to have the subroutine return some information about the execution of the subroutine. For example, what use would it be to have a subroutine that calculates the average of two numbers if we never found out what the average was? We could of course print the average in the subroutine, but it is often more useful to return the value to the "asker" as return value. In this case the subroutine could also be used in a situation where we don't want to print the average but rather use it in another equation. Returning a value can be performed with the return statement, and the return statement always ends he execution of the subroutine (i.e. the execution returns to the part in the program that made the subroutine call).

A subroutine that returns a value is usually called a function.

9.1 A function for calculating averages

The contents of this chapter on video (in Finnish):

Before implementing the function we plan that it will be called as follows:

        double average;
        average = Average(3, 4);

This means that when we return from the subroutine, it returns the result it calculated and the result is assigned to a variable.

Now we will implement the function.

The first line declares the public and static subroutine. In the Snowman example, the word void followed the word static, which meant that the subroutine returned no value. Now, because we want the subroutine to return the average of the numbers it received as parameters, we must write the type of the return value instead of the word void after the word static. Because the average of two integers can be a decimal number, the type of the return value is double. The parameters are given in parentheses. Now the parameters are the two integers a and b. The second line declares the real number variable average. The third line assigns the sum of a and b divided by two to the variable average. The fourth line returns the value of the average variable.

9.2 Calling functions

Subroutines can now be used in the main program like below for example.

The call can also be written in short:

Because the Average subroutine always returns a double type floating point number, the call can be used like any other value of the double type. It can for example be printed or stored in a variable.

The animation below contains two consecutive calls, which demonstrate subroutine calls with different values. The latter call demonstrates how the value of an expression is calculated in a subroutine call. In other words, the function (or any other subroutine) call can contain any type of expressions that produce a value that can be assigned to the corresponding parameter. In this case, 2+6 is an expression that produces an int value and the corresponding parameter in the subroutine, called b, is also of the type int. We will learn later that an expression can also contain function calls.

9.3 Implementing the function in another way

In fact, the entire Average subroutine could be shorter:

At its simplest, the Average subroutine could even be coded as follows.

All the different methods above are correct, and it cannot be said which one is the best. Sometimes writing the "intermediary" phases clarifies the code, but in the case of the Average subroutine, the last method is the clearest and shortest.

If the function needs to be debugged, it is easier if there are intermediary results that are assigned to variables. In this case, a one-line function may need to be pieced.

One purpose of tests is to make sure that even if we change the implementation, the correctness of the result is easy to check. However, we need to remember that tests may not ensure that the function works in each case! View the tests in the previous example by clicking Show all code and run the tests by clicking Test. See the generated documentation by clicking Document.

9.4 Several return statements

A subroutine can also contain several return statements. An example of this can be found in section 13.5.1. If the code contains several return statements, the "extra" statements must be executed conditionally.

However, code that contains several return statements is usually risky. A good example is when a code that calculates something and returns it has been written first:

   ...
   if ( a < 0 ) return sum / number;
   ...
   return sum/number;

When the code has been tested with several values, we notice that number can be zero, and we change the code:

   ...
   if ( a < 0 ) return sum / number;
   ...
   if ( number == 0 ) return 0;
   return sum/number;

What goes wrong here? The fact that even the first return statement fulfills the condition that number is zero.

It is up to you if you want to avoid multiple returns or not. Multiple return statements can make your code clearer because it allows to avoid blocks within blocks.

9.5 The function returns one value

A subroutine cannot have multiple return values at a time per se. However, it can for example return an array which contains several values. Another method is to create and return an object that has several attributes (values). There is a third way in C#, which will not be taught in this course: ref and out parameters (however, the out parameter was used in the TryParse function), see:

Methods and subroutines that take in parameters and return a value are called functions. The name is actually quite descriptive if you compare the Average subroutine to the mathematical function \(f(x, y) = (x + y) / 2\).

Functions should always work by using the information they receive as parameters alone (and not require any other information about the program). Also, changing the values received as parameters should be avoided. In purely functional way of thinking programming, functions have no side effects. Side effects include for example printing on screen or changing the program status. In object-oriented way of thinking programming (and on this course as well) this requirement cannot be fully accomplished. For example, functions in Jypeli games often change the game status by for example adding a new object to the game level, which means that they are not completely free of side effects.

9.6 Function calls have a cost

What is the difference between this:

and this:

in regard of code execution?

In the first one, the average of 2 and 5 is only calculated once, after which the result is stored in a variable. Printing uses the result that was stored.

The latter version calculates the average of 5 and 2 at the time of printing. In other words, the result is calculated twice. Even thought the latter method saves one line of code, it uses the computer's resources in vain by calculating the same expression twice. In this case it doesn't matter too much, because executing the Average subroutine doesn't strain modern computers much. However, it would be useful to learn to code so that no extra actions are performed in vain.

9.7 CircleArea, an example of a one-parameter function

In the previous example, the function received two parameters. The number of parameters depends on the needs of the function, and the number can be anything from 0 to n. However, functions that don't take in any parameters are quite rare.

The following is an example of a one-parameter function:

Remember that the previous function could be called in any of the following ways (try in the example):

  ...
  double radius = 2.1;
  area = CircleArea(radius);  // with a variable
  ...
  area = CircleArea(radius + 7.0); // and with any expression that results in a double
  ...
  CircleArea(12);  // this call can be used as well, but it doesn't make any sense
                   // because the result is not handled in any way here.    

9.8 Tasks related to functions

9.8.1 Exercise

At the end of the Variables chapter we made a program that calculated the BMI of a person. Make a new version of the program that calculates the BMI in a function. The function receives the height and weight as parameters and returns the BMI. Printing the result is performed in the main program.

9.8.2 Exercise

We have the following program, in which the implementation of the subroutine is still in progress. XXX YYY ZZZ TriangleArea(??? number1, IIII number2) {} . Think about the answers to the following questions and then watch the video to see the right answer. Complete the program to work.

9.8.2.1 Answer options

9.8.2.2 Questions and answers

10. Integrated Development Environment (IDE)

10.1 Git

Käytännön ohjelmistokehitystyössä ohjelmistojen kehittämiseen osallistuu aina useampi henkilö yhteistyönä. Yhtenäisten koodien varmistamiseksi käytetään niin kutsuttuja versionhallintatyökaluja, joista nykyisin yleisimmin käytetty on Git. Git on kehitetty Linus Torvaldsin toimesta.

Git-versiohallinnan pääidea on mahdollistaa ohjelmakoodiin tehtyjen muutosten seuranta ja hallinta. Versiohallintaan tallennetaan ohjelmistoon tehdyt muutokset aikajärjestyksessä. Tämä mahdollistaa yhteistyön useiden kehittäjien kesken samanaikaisesti ja tarjoaa mahdollisuuden palata aiempiin versioihin tarvittaessa. Git tallentaa kunkin kehittäjän tekemät muutokset erillisinä "commiteina", jotka voidaan yhdistää pääkehityshaaraan ("main"), tai ns. haarojen ("branch") kautta, mikä helpottaa uusien ominaisuuksien lisäämistä ja virheiden korjaamista eristyksissä.

Tällä kurssilla käytämme vain muutamia Git-versiohallinnan ominaisuuksia, mutta suosittelemme tutustumaan myös muihin ominaisuuksiin, kuten haarojen käyttöön, mikäli aiot jatkaa ohjelmistokehityksen parissa.

Netissä on lukuisia palveluita, joissa voidaan säilyttää ja julkaista Git-versiohallintaa käyttäviä projekteja. Eräitä tunnettuja ovat GitHub ja GitLab. Näihin palveluihin on rakennettuna myös tikettijärjestelmä, johon lisätään havaittuja bugeja ja kehitysehdotuksia kortteina. Kehittäjä valitsee näistä korteista yhden tai useampia ja työskentelee niiden parissa. Kun kortti on valmis, se siirretään valmiiden korttien joukkoon, ja haarassa oleva koodi yhdistetään versiohallinnan pääkehityshaaraan. Tikettijärjestelmä ei siis ole osa Git-versiohallintaa, vaan yksi lisäpalvelu käytettäväksi Gitin ohessa. Git on ilmainen, mutta lisäpalvelut usein maksavat.

Voit tarkastella esimerkiksi TIMin GitHubiin kirjattuja tikettejä tästä linkistä.

Ohjeet Git-versiohallinnan asentamiseksi ja käyttämiseksi tällä kurssilla löydät työkalut-sivulta.

10.2 Integroitu kehitysympäristö, IDE

Even though coding can be done even in a simple text editor, the programmer might start needing more features from the development tool as the size of the program grows. IDEs, Integrated Development Environments, offer more features than basic editors.

IDEs combine a set of individual tools such as:

• text editor (that usually understands the target language better than a regular editor
• compiler
• linker tools
• version management tools
• debugger

Free IDEs for C# include for example

These instructions have been tested for the Visual Studio 2015 Community version in the Windows environment.

Kaikenlaiset pilvipalvelut ovat yleistyneet, ja myös pilvipohjaisia kehitysympäristöjä on olemassa. Kuitenkin edelleen yleinen käytäntö ohjelmoinnin opiskelussa, kuten myös Ohjelmointi 1 -kurssilla, on asentaa kehitysympäristö omalle paikalliselle tietokoneelle. Tämä lähestymistapa tarjoaa useita merkittäviä etuja.

• Suorituskyky: Paikallisella asennuksella hyödynnät tietokoneesi tehon täysimääräisesti, mikä yleensä tarkoittaa nopeampaa koodin kääntämistä, suorittamista ja vianmääritystä verrattuna etäympäristöihin.
• Täysi kontrolli: Sinulla on täysi hallinta asetuksista ja konfiguraatiosta. Voit mukauttaa ympäristöä omiin tarpeisiisi ja työskennellä ilman riippuvuutta ulkopuolisista palveluista.
• Hinta: Pilvipohjaiset kehitysympäristöt, jotka täyttävät Ohjelmointi 1 -kurssin osaamistavoitteet, kuten debuggaus, eivät yleensä ole ilmaisia. Omalle koneelle asennettu kehitysympäristö on maksuton.
• Toimialan standardi: Paikallisen kehitysympäristön asentaminen vastaa alan normeja ja toimintatapoja.

10.3 Usage

10.3.1 Coding a program

First, change the name of the code file to something more descriptive by right-clicking the file in `Solution Explorer´ and selecting Rename. When the name of the file has been changed, Visual Studio asks if you want to roll the changes in other parts of the program as well. Answer Yes.

10.3.2 Compiling and running a program

When the program code has been written, compiling and running the program can be done by clicking the Debug button (green triangle, a "play" button) or by clicking F5 (debug). When you click this button, Visual Studio will compile the program automatically and execute it in the so-called debug mode. The program can also be executed without debugging by pressing Ctrl+F5 (start without debugging). However, during the development of a program it is often useful to run the program specifically in debug mode so that possible run-time errors can be viewed in Visual Studio. On the other hand, command prompt programs "stop" ("Press any key to continue...") after the execution of the program if run without debug mode, which can sometimes be a more useful option, especially with very small programs.

If we want to stop the execution of a program for some reason, this can be done by pressing Shift+F5.

10.4 Debugging

Errors in program code are an unfortunate reality. Bugs will be part of your life forever no matter how good of a programmer you are. A good programmer acknowledges this fact and prepares for this by acquiring tools for tracing and fixing errors. There are small errors that do not affect the function of a program in any way (like a spelling mistake in a button), but there are also critical errors. These types of errors crash the program. Syntax errors can be small, but they prevent the compilation of a program. Logical errors won't be caught by the compiler, but they might cause runtime errors.

For example, your program might not be able to enter the correct data to database because a necessary field is missing or the data might be entered incorrectly in some circumstances. These kinds of errors related to some fault in application logic might be either semantic errors or logical errors.

Finding logical errors can be hard, especially with more complex programs, because the program might not in any way indicate that there is an error - you only notice the error from the end result.

This is where the error tracking function, the bebugging tool in VS comes in handy. With debugging, you can follow the execution of the program line by line, also keeping track of how the values of variables change. This helps significantly in finding the reason for an error or unwanted functionality. Another traditional way to do this is to add print statements in the program, but these might accidentally end up in the final version and even change the functionality of the program.

In VS, debugging starts by setting a breakpoint somewhere in the program. A breakpoint is the point in code where we want to halt the execution of the program temporarily. Once we halt the program, we can execute it statement by statement. Therefore, we have to set the breakpoint somewhere before the supposed error point. If we want to debug the entire program, we set the breakpoint at the beginning of the program. Set a breakpoint to where the cursor is currently by clicking F9 or by clicking the grey area left of the line numbers in the code window. The breakpoint appears as a red circle and the (first) statement on the line is highlighted red.

When the breakpoint has been set, click the Debug button at the top of the screen or press F5. In Visual Studio, running the program and debugging the program is the same thing. Of course the program can be run without debug mode as well, but then the possible errors in the program are not caught by Visual Studio, and dividing by zero for example will crash the program.

Execution of the program has now stopped at the point where we set the breakpoint. Open the Locals tab (at the bottom of the screen) unless opened already. In debug mode, the Locals panel shows all the (local) variables that are currently in scope and their values. The centre of the screen contains the program code and the line where the execution currently is is highlighted in yellow. There is also a yellow arrow to the left which points the current line number.

There are two commands for executing the program line by line: Step Into (F11) and Step Over (F10). The commands are quite similar, but if the statement is a subroutine call, the Step Into command moves execution to the first line of the subroutine whereas the Step Over command executes the subroutine call as if it was a single statement. All the variables currently in scope and their values are visible on the Variables tab to the right.

When we no longer want to execute the program line by line, we can either execute the rest of the program by clicking Debug ? Continue (F5), or we can end the execution of the program immediately by clicking Terminate (Shift+F5).

Termi debug johtaa yhden legendan mukaan aikaan, jolloin tietokoneohjelmissa ongelmia aiheuttivat releiden väliin lämmittelemään päässeet luteet. Ohjelmien korjaaminen oli siis kirjaimellisesti hyönteisten (bugs) poistoa. Katso lisätietoja Wikipediasta:

http://en.wikipedia.org/wiki/Software_bug#Etymology.

• lue lisää debuggauksesta
  • sivulla on kerrottu myös vastaavat Mac painkikkeet

10.5 Useful features

10.5.1 Search for syntax errors

Visual Studio spots the majority of syntax errors, so a large number of errors can be fixed already before compilation - more specifically, VS compiles code all the time to spot and notify about all possible errors. When VS finds an error, an error message appears in the Error List panel with the line number where the erroneous code is located. Additionally, if the error can be located, VS will underline the erroneous strip of code with red wavy underscore. If you hover the cursor over the red cross, a specification of the error in question is shown. Notice that VS might not locate the error exactly right every time. The error might also be in the previous or following line.

10.5.2 Code completion, IntelliSense

IntelliSense is one of the best features in VS. IntelliSense is a versatile automatic code completion tool and documentation interpreter.

One documentation-based feature in IntelliSense is the browsing of parameter lists in overloaded subroutines. Write for example

string name = "Kalle";

After this, write name and a dot ".", and a list of the functions and methods that the object can use will appear. After selecting a subroutine from the list, click the parentheses: this allows you to browse the different "versions" of the subroutine, i.e., the functions with the same name but different number of parameters. In addition, you get a short description of the functionality of the method and even examples of its use.

IntelliSense also allows you to write faster and especially to avoid spelling errors. You can work so much faster if you use the Ctrl+Space shortcut while you write code. By pressing Ctrl+Space, VS will attempt to complete the statement based on what you have written so far. If there are multiple options, VS will show a list of all the available options.

10.5.3 Automatic formatting

Visual Studio tries to format the code "neatly" while you write code. However, the user can either intentionally or accidentally break the internal formatting settings in VS. In this case, the code can be formatted correctly again automatically by clicking Edit/Advanced/Format Document or by pressing Ctrl+E,D (hold Ctrl, click E first and then D).

10.5.4 Task List

If the comment lines in the code read TODO:, VS will put together a task list, which shows the line comment, project, file, and line number. You can open the task list from View > Task List.

Double-clicking a "task" in the Task List will move focus on the line that contains the text TODO, which makes it easy to return to tasks later. This can be utilised in case implementing something is interrupted for some reason.

10.6 More information about the use of Visual Studio

The course extra material page contains more tips and instructions for using Visual Studio (currently only available in Finnish):

• Creating a project in Visual Studio (click here for English)
• Tips for using Visual Studio
• Xamarin projects in Visual Studio
• Debugging (click here for English)

11. Testing

“Program testing can be a very effective way to show the presence of bugs, but is hopelessly inadequate for showing their absence.” - Edsger W. Dijkstra

Testing a program means inspecting the quality or flawlessness of a program. Testing can be done by runnign the program as is, for example by trying out different types of usage or by printing the status of a variable on screen or by investigating if the program works as expected. However, testing even the most simple program like this would take a lot of time.

Printing or running the program would have to be done again each time the programmer changes the code. This is because we cannot possibly know whether the tests we did before the change will work similarly after the change. One technique that eases testing is unit testing. The idea behind unit testing is that each component, such as a subroutine or a method has its own test which can all be run at the same time. In this way, we can execute all the tests (that only need to be written once) again after each small change we do.

One notable style of design is TDD Test Driven Development. Its idea is that before we write the code we think about how to test the functionality, preferably with automated tests. In this way, thinking about testing gives direction to the planning and coding. Combining writing tests beforehand and unit tests, we can produce higher quality code according to modern standards.

11.1 Comtest

Visual Studio allows writing unit tests. The problem is that writing these VS-supported unit test files is quite laborous. To make unit testing easier, the department of information technology in Jyväskylä has developed the Comtest tool, which utilises the internal testing system in Visual Studio, but considerably lowers the threshold to write unit tests in programs.

The idea of the Comtest testing tool is that tests are written within the documentation comments in subroutines and methods with a simple syntax and then the tool automatically generates the test projects and test files themselves. The tests also act as examples of the functionality of the subroutine or method. Because writing the tests is so easy with Comtest, this is in favour of writing tests while we think about what the function should do e.g. with the value of each parameter. This allows us to achieve the goals of TDD more efficiently.

The installation instructions for ComTest can be found from:

• https://tim.jyu.fi/view/kurssit/tie/ohj1/tyokalut/rider.

The phases of writing and testing subroutines was discussed in chapter Coding subroutines. Revise the steps!

11.2 Usage

In order to use ComTest, the class in which we want to test subroutines needs to be public, otherwise testing cannot be completed. Similarly, each subroutine that is tested needs to be a public subroutine.

Write documentation comments in the subroutine. Then move to the end of the documentation comment, add one empty line (without slashes), write comt, and press Tab+Tab (Press Tab two times). Visual Studio will automatically generate a template for writing the tests. The following lines should appear in the documentation comments.

The tests are written within the pre tags. The syntax above is for the Doxygen tool (and other automatic documentation tools).

Subroutines and methods can be tested simply by giving them parameters and writing the expected outcome with the given parameters. ComTest utilises a specific comparison operator, which consists of three equals signs (===). This means that the value must be of the same type and have the same content. When testing real numbers (e.g. float types), the test has to be written with the comparison operator "almost equal" (~~~). Due to an error in Doxygen, there must be an even number of ~~~ test lines in order to generate documentation.

Notice that subroutine calls in ComTest need to contain the name of the class before the name of the subroutine. In the example below, the class name is Calculations.

When doing TDD correctly, tests are written before implementing the subroutine itself. First we will write a subroutine stub (minimal code with the correct syntax). Then we write the tests and check that they return red (i.e. are not passed). When the tests return red, we can implement the subroutine to work as planned, after which the tests should return green.

With the help of tests we can plan any special cases that need to be taken into account in the subroutine and how to act with them. For example, the average of elements in an empty array. When the tests are in the subroutine comments, the documentation comments about special cases can be left out because the course of action for them is clear from the test examples.

• See also additional examples of ComTests.

11.2.1 Write a stub implementation

In order to test the syntactic function of tests and their ability to detect errors, we first write a stub of the subroutine. A stub is a syntactically correct subroutine, but it doesn't solve the task it's given, not at least for all the possible test values. A stub for a void subroutine can simply be an empty subroutine. For a function, the stub can be a return statement which returns an expression of any value that matches the function return type. For an integer function, 0 is a good return value, for example.

Examples of stubs:

  return;                          // for a void subroutine
  return 0;                        // int or double type functions
  return "";                       // string type functions
  return new StringBuilder("");    // StringBuilder type functions
  return object;                   // for a function that receives an object as 
                                   // parameter and needs to return an object of the
                                   // same type.
  return null;                     // this can be used for object types as well
                                   // (including strings, arrays, StringBuilders)
  return new int[0];               // returns an empty int array 

Let's write for example a Combine subroutine that combines the numbers of two non-negative numbers given. First, we write the introductory line for the subroutine, braces, and a stub between the braces:

public static int Combine(int a, int b)
{
   return 0;
}

11.2.2 Write documentation and tests

Next, add the documentation of the subroutine (in Visual Studio you can simply write /// above the introductory line of the subroutine to generate the template).

Then write the tests for the subroutine.

In the previous example, view the generated documentation by clicking the link Document. Then click the class name Calculations and the link Combine. You can now view the examples generated from the subroutine documentation.

Now when we run the test above (click the Test button), we receive the message that the test on line

24     /// Combine(1, 0) === 10;

fails, because the value 10 was expected, but the value 0 was returned. Now we know that the tests can detect at least some of the cases where the subroutine works wrong. These types of tests based on examples unfortunately cannot detect all possible erroneous functionality in a subroutine.

11.2.3 Implement the subroutine and run the tests

When we have a syntactically correct subroutine and its stub, we can write the implementation of the subroutine (a function in this case), which should accomplish the intended task and pass the test cases.

In this subroutine implementation there is the (naive) assumption that the numbers given as parameters fulfill the condition (non-negative). Later we will learn how to handle situations where this condition is not fulfilled.

The implementation above is perhaps not the most efficient, but with the help of tests it can be improved and still check the correct functionality quickly. Now run the tests by clicking the Test button.

Try changing the implementation so that you replace the return statement with:

return result+1;

Now rerun the tests. Restore the original format and rerun the tests again.

Let's study the tests in more detail,

Combine(0, 0) === 0;

The line above tests that if the Combine subroutine receives the values 0 and 0 as parameters, it should also return the value 0.

Combine(1, 0) === 10;

Next we will test the case where if the first parameter is the number 1 and the other is the number 0, the combination is 10, so the subroutine should return the number 10. The combination of zero and one would be 01, but the equivalent number is of course 1, so 1 is the number that we expect the subroutine to return, and so forth.

The Visual Studio test project can now be generated and run by pressing Ctrl+Shift+Q or clicking Tools/ComTest. If the Test Results tab (at the bottom of the screen by default, appears when you run ComTest) appears green and reads Passed, the tests passed correctly. In the case of a red circle the tests either didn't pass or the test file contains errors.

11.2.4 General about tests

Tests are basically just like any other part in a subroutine where calls to the tested subroutine are executed. In the example above, the test cases contain only one line, but one test case can of course consist of multiple lines as well, in which the test parameters are first set, then the subroutine call is made, and finally the results are inspected with the "test operators" === or ~~~ to see if everything worked as planned. For example, the tests for the Combine function could be written on multiple lines as well. Below are a few examples of writing the same tests more widely. However, if the same thing can easily be written on one line, it is often easier to read more compact test cases.

Later when discussing arrays, lists, and the StringBuilder class we will present examples where test code has been divided on several lines.

In practice, running ComTest means that the test code in the comments is formulated into a "regular" subroutine (NUnit test methods) and all the tests in the file is formulated into one test class (CombineTest.cs in the example). Then, this test class is run with the NUnit test environment so that the environment calls each test subroutine and executes the code there. If some condition is not fulfilled, the result will appear in red. After you have learned more about ComTest and testing, you can look at what the Test files contain.

Also tests need to be tested. It might be that our tests contain errors as well. A part of this test is done when testing the stub. You might want to try writing an error in the tests on purpose. In this case, the tests should of course appear red. If not, one of the tests is wrong or there is an error in the subroutine.

Writing good tests is a skill on its own. All possible situations cannot be tested, so we have to select which parameters to test. At the very least, it is best to test the most probable error spots, which are usually at least any "borderline situations".

Typical borderline situations are for example when the largest element in an array is located at the beginning, centre, or end of the array. Usually also arrays with one element or empty arrays (or strings) are special cases worth testing. In addition, when searching for the location of the largest element it can be useful to test the situations where there are multiple elements with the same size as the largest element.

The borderline cases in the Combine subroutine in the example are zeros, on each side separately and combined. Otherwise the test values have been selected pretty much at random. Additionally, it would be useful to add tests and handling for negative numbers.

11.3 Testing floating point numbers

Floating point numbers (double and float) are tested with the ComTest comparison operator with three tildes (~~~). This is because all real numbers cannot be presented accurately on a computer, so we need to allow a small error margin between the expected value and the actual result. Let's make a subroutine called Average that calculates the average of two double numbers, and write documentation comments and Comtest tests for it at the same time.

Because of an error in the Doxygen program, there needs to be an even number of ~~~ test lines in order to generate the documentation of the tested function.

By default, the error margin (comparison accuracy) is six decimals. The error margin can be changed with the #TOLERANCE definition:

12. Strings

In this chapter, we will discuss strings in detail. Strings can be divided to immutable and mutable strings. In C#, an immutable string is of the type String, which we already introduced when we discussed objects. Immutable strings cannot be changed after creation. Handling a mutable string, a StringBuilder object is more sensible in some situations. Even though String objects cannot be changed, they are useful enough in certain situations.

12.1 Initialisation

We already saw some examples of String objects when the discussed objects. A String can be initialized in two ways:

The latter way resembles initialising primitive data types, but we must still remember that strings in C# are always objects. We can have for example two string variables (i.e. references) that both point to the same string. On the other hand, we might also have different references that point to different strings with the same content.

Seuraavassa animaatiossa on vielä näytetty mitä tapahtuu jos jono1-merkkijonoa "muutetaan". Merkkijonohan on muuttumaton ja siksi rivi

jono1 += " istuu";

on sama kuin

jono1 = "Kissa" + " istuu";

jolloin syntyy uusi merkkijono-olio, johon jono1 käännetään viittaamaan.

The example program forces to create a new string for the variable jono4 because otherwise the user would take advantage of immutability of strings and assign the same reference to jono4 as was assigned to jono3.

Thanks to the immutability of the string object strings can be thought to behave like regular variables. For example, if a subroutine call takes a string as parameter, it will still have its original value after returning from the subroutine. This allows us to not constantly think of strings as references and objects. This is not true for mutable strings (StringBuilder)

For example if an integer variable has the following assignment:

int number = 5;

we say that the value of number is 5. If a string variable has been assigned

string str = "Cat";

we should technically say that the value of str is a reference to an object that contains the letters Cat. Instead we somewhat erroneously say that the value of the string is Cat.

12.1.1 A string is like an array

Because a string is similar to an array, we can take one character from it just like from the elements in an array:

Be careful not to point to an index that is not in the array, or in this case, in the string. For example:

Running this kind of program would result in an exception:

Unhandled Exception:
System.IndexOutOfRangeException: Array index is out of range.
  at Pohja3.Main () [0x00000] in <filename unknown>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.IndexOutOfRangeException: Array index is out of range.
  at Pohja3.Main () [0x00000] in <filename unknown>:0 

12.1.2 Be careful of empty strings

Because a string is an object, the equivalent variable is a reference and a reference can also be a null reference. In this case the string might be empty, i.e. the string contains no characters. This is different from a string that contains spaces (blank characters).

An empty string can actually be useful, but you also need to be careful not to include any characters in it, not even one.

The emptiness of a string can be tested by comparing its length. However, there is the risk that the reference itself is null, which might result in a NullReferenceException and crash the program. Only if we can be certain that the reference cannot be null, testing the length is ok. This can be tested with the static function IsNullOrEmpty in the String class (also works with null references) or with a combined condition statement.

12.2 Useful methods and features

The String class contains a lot of useful methods, some of which will be discussed now. All methods can be found in the C# MSDN documentation.

12.2.1 Methods return a new string

Note that the methods in the String class that return a string actually create a new string, i.e. return a reference to this new object.

For example ToLower() return the reference to a new string which has all the letters of the string in lower case. Here, as in any of the other similar methods, the original string doesn't change at all. The method creates a new object with the same letters as the original string, but in lower case. Then the reference to this new string is returned. The original string remains unchanged (immutable).

The value of the reference variable k in the example above can also be replaced while assigning. In this case, the original object that contains the string "Cat" becomes garbage (unless another reference variable points to it).

Assigning the value to the auxiliary value k is not necessarily needed because the object created when calling the ToLower method can be used as part of an expression, for example as the argument of the WriteLine method. Note however that in the example below the k variable still refers to the string "Cat" (with a capital C).

Remember that the object for which the conversion is made doesn't necessarily need its own variable; the example below converts the string "Dog".

Because the original string remains unchanged, it's not useful to make the call without assignment. Example below.

The animation below demonstrates how the ToLower method creates a new string: