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.
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:

#include
<stdio.h>
int main()
{
printf( "I am alive! Beware.\n" );
getchar();
return 0;
}


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 an gain access to many different functions. For example, the printf function requires getchar. 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 imporant 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 <variable type> <name of variable>;. (The brackets here indicate that your replace the expression
with text described within the brackets.) For instance, a basic variable declaration might look like this:

int myVariable;

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:

int x;
int a, b, c, d;
char letter;
float the_float;

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:

Wrong

#include <stdio.h>
int main()
{
/* wrong! The variable declaration must appear first */
printf( "Declare x next" );
int x;
return 0;
}
Fixed

#include <stdio.h>
{
int x;
printf( "Declare x first" );
return 0;
}


Reading Input:

Using variables in C for input or output can be a bit of a hassle at first, but bear with it and it will make sense. We'll be using the
scanf function to read in a value and then printf to read it back out. Let's look at the program and then pick apart exactly what's going on.
You can even compile this and run it if it helps you follow along.

#include <stdio.h>
int main()
{
int this_is_a_number;
printf( "Please enter a number: " );
scanf( "%d", &this_is_a_number );
printf( "You entered %d", this_is_a_number );
getchar();
}


So what does all of this mean? We've seen the #include and main function before; main must appear in every program you intend to
run, and the #include gives us access to printf (as well as scanf). (As you might have guessed, the io in stdio.h stands for "input/output";
std just stands for "standard.") The keyword int declares this_is_a_number to be an integer.
This is where things start to get interesting: the scanf function works by taking a string and some variables modified with &. The string tells
scanf what variables to look for: notice that we have a string containing only "%d" -- this tells the scanf function to read in an integer. The second
argument of scanf is the variable, sort of. We'll learn more about what is going on later, but the gist of it is that scanf needs to know where the variable
is stored in order to change its value. Using & in front of a variable allows you to get its location and give that to scanf instead of the value of the variable.
Think of it like giving someone directions to the soda aisle and letting them go get a coca-cola instead of fetching the coke
for that person. The & gives the scanf function directions to the variable.
Using variables in C for input or output can be a bit of a hassle at first, but bear with it and it will make sense. We'll be using the
scanf function to read in a value and then printf to read it back out. Let's look at the program and then pick apart exactly what's going on.
You can even compile this and run it if it helps you follow along.

#include <stdio.h>
int main()
{
int this_is_a_number;
printf( "Please enter a number: " );
scanf( "%d", &this_is_a_number );
printf( "You entered %d", this_is_a_number );
getchar();
}

So what does all of this mean? We've seen the #include and main function before; main must appear in every program you intend to
run, and the #include gives us access to printf (as well as scanf). (As you might have guessed, the io in stdio.h stands for "input/output";
std just stands for "standard.") The keyword int declares this_is_a_number to be an integer.
This is where things start to get interesting: the scanf function works by taking a string and some variables modified with &. The string tells
scanf what variables to look for: notice that we have a string containing only "%d" -- this tells the scanf function to read in an integer. The second
argument of scanf is the variable, sort of. We'll learn more about what is going on later, but the gist of it is that scanf needs to know where the variable
is stored in order to change its value. Using & in front of a variable allows you to get its location and give that to scanf instead of the value of the variable.
Think of it like giving someone directions to the soda aisle and letting them go get a coca-cola instead of fetching the coke
for that person. The & gives the scanf function directions to the variable.


Here are a few examples:

a = 4 * 6; /* (Note use of comments and of semicolon) a is 24 */
a = a + 5; /* a equals the original value of a with five added to it */
a == 5 /* Does NOT assign five to a. Rather, it checks to see if a equals 5.*/
The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is extremely useful in many areas of C; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how < and > function. They are greater than and less than operators.
For example:

a < 5 /* Checks to see if a is less than five */
a > 5 /* Checks to see if a is greater than five */
a == 5 /* Checks to see if a equals five, for good measure */