The basics of C
This tutorial is a port of the C++ tutorial but is designed to be a stand-alone introduction to C, even if you've never programmed before. Unless you have a particular reason to learn C instead of C++, I recommend starting the C++ tutorial instead. Nevertheless, if you do not desire some of C++'s advanced features or simply wish to start with a slightly less complicated language, then this tutorial is for you!
Getting set up
C is a programming language of many different dialects, similar to the way that each spoken language has many different dialects. In C, dialects don't exist because the speakers live in the North or South. Instead, they're there because there are many different compilers that support slightly different features. There are several common compilers: in particular, Borland C++, Microsoft C++, and GNU C. There are also many front-end environments for the different compilers--the most common is Dev-C++ around GNU's G++ compiler. Some, such as GCC, are free, while others are not. Please see the compiler listing for more information on how to get a compiler and set it up. You should note that if you are programming in C on a C++ compiler, then you will want to make sure that your compiler attempts to compile C instead of C++ to avoid small compatibility issues in later tutorials.
Each of these compilers is slightly different. Each one should support the ANSI standard C functions, but each compiler will also have nonstandard functions (these functions are similar to slang spoken in different parts of a country). Sometimes the use of nonstandard functions will cause problems when you attempt to compile source code (the actual C code written by a programmer and saved as a text file) with a different compiler. These tutorials use ANSI standard C and should not suffer from this problem; fortunately, since C has been around for quite a while, there shouldn't be too many compatibility issues except when your compiler tries to create C++ code.
If you don't yet have a compiler, I strongly recommend finding one now. A simple compiler is sufficient for our use, but make sure that you do get one in order to get the most from these tutorials. The page linked above, compilers, lists compilers by operating system.
Every full C program begins inside a function called "main". A function is simply a collection of commands that do "something". The main function is always called when the program first executes. From main, we can call other functions, whether they be written by us or by others or use built-in language features. To access the standard functions that comes with your compiler, you need to include a header with the #include directive. What this does is effectively take everything in the header and paste it into your program. Let's look at a working program:
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Let's look at the elements of the program. The #include is a "preprocessor" directive that tells the compiler to put code from the header called stdio.h into our program before actually creating the executable. By including header files, you can gain access to many different functions--both the printf and getchar functions are included in stdio.h. The semicolon is part of the syntax of C. It tells the compiler that you're at the end of a command. You will see later that the semicolon is used to end most commands in C.
The next important line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The "curly braces" ({ and }) signal the beginning and end of functions and other code blocks. If you have programmed in Pascal, you will know them as BEGIN and END. Even if you haven't programmed in Pascal, this is a good way to think about their meaning.
The printf function is the standard C way of displaying output on the screen. The quotes tell the compiler that you want to output the literal string as-is (almost). The '\n' sequence is actually treated as a single character that stands for a newline (we'll talk about this later in more detail); for the time being, just remember that there are a few sequences that, when they appear in a string literal, are actually not displayed literally by printf and that '\n' is one of them. The actual effect of '\n' is to move the cursor on your screen to the next line. Again, notice the semicolon: it is added onto the end of all lines, such as function calls, in C.
The next command is getchar(). This is another function call: it reads in a single character and waits for the user to hit enter before reading the character. This line is included because many compiler environments will open a new console window, run the program, and then close the window before you can see the output. This command keeps that window from closing because the program is not done yet because it waits for you to hit enter. Including that line gives you time to see the program run.
Finally, at the end of the program, we return a value from main to the operating system by using the return statement. This return value is important as it can be used to tell the operating system whether our program succeeded or not. A return value of 0 means success.
The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .c file, and then compile it. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile. Otherwise compiling and running should be as simple as clicking a button with your mouse (perhaps the "build" or "run" button).
You might start playing around with the printf function and get used to writing simple C programs.
Explaining your Code
Comments are critical for all but the most trivial programs and this tutorial will often use them to explain sections of code. When you tell the compiler a section of text is a comment, it will ignore it when running the code, allowing you to use any text you want to describe the real code. To create a comment in C, you surround the text with /* and then */ to block off everything between as a comment. Certain compiler environments or text editors will change the color of a commented area to make it easier to spot, but some will not. Be certain not to accidentally comment out code (that is, to tell the compiler part of your code is a comment) you need for the program.
When you are learning to program, it is also useful to comment out sections of code in order to see how the output is affected.
Using Variables
So far you should be able to write a simple program to display information typed in by you, the programmer and to describe your program with comments. That's great, but what about interacting with your user? Fortunately, it is also possible for your program to accept input.
But first, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables; when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float. Each type can store different types of data.
A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable. Some variables also use more of the computer's memory to store their values.
It may seem strange to have multiple variable types when it seems like some variable types are redundant. But using the right variable size can be important for making your program efficient because some variables require more memory than others. For now, suffice it to say that the different variable types will almost all be used!
Before you can use a variable, you must tell the compiler about it by declaring it and telling the compiler about what its "type" is. To declare a variable you use the syntax
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Note once again the use of a semicolon at the end of the line. Even though we're not calling a function, a semicolon is still required at the end of the "expression". This code would create a variable called myVariable; now we are free to use myVariable later in the program.
It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. If you attempt to use an undefined variable, your program will not run, and you will receive an error message informing you that you have made a mistake.
Here are some variable declaration examples:
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While you can have multiple variables of the same type, you cannot have multiple variables with the same name. Moreover, you cannot have variables and functions with the same name.
A final restriction on variables is that variable declarations must come before other types of statements in the given "code block" (a code block is just a segment of code surrounded by { and }). So in C you must declare all of your variables before you do anything else:
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Fixed
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