We will walk through the entire language and show you how to become a C programmer starting at the beginning. You will be amazed at all the different things you can create once you know C! The following articles will lead your through a complete and thorough introduction to the language.

What is C?

C is a Computer Programming Language. That means that you can use C to create lists of instructions for a computer to follow. C is probably the most popular and widely used programming language on the planet. It has been around for several decades and has won widespread acceptance because it gives programmers maximum control and efficiency. C is an easy language to learn. C is a bit more cryptic in its style than some other languages, but you get beyond that fairly quickly. If you were going to learn just one programming language, C would give you the most bang for your buck because it is so popular.

C is what is called a compiled language. This means that once you write your C program, you must run it through a C compiler to turn your program into an executable that the computer can run (execute). The C program is the human-readable form, while the executable that comes out of the compiler is the machine-readable and executable form. What this means is that to write and run a C program you must have access to a C compiler. If you are using a UNIX machine (for example, if you are writing CGI scripts in C on your host's UNIX computer or if you are a student working on a lab's UNIX machine), the C compiler is available for free. It is called either "cc" or "gcc" and is available on the command line. If you are a student, then the school will likely provide you with a compiler - find out what they are using and learn about it. If you are running at home on a Windows machine, you are going to need to download a free C compiler or purchase a commercial compiler. The most widely used commercial compiler is Microsoft's Visual C++ environment (it compiles both C and C++ programs). Unfortunately this program costs several hundred dollars. If you do not have hundreds of dollars to spend on a commercial compiler, then you can use one of the free compilers available on the web. See http://delorie.com/djgpp/ as a starting point in your search.

In the next article we will start at the beginning with an extremely simple C program and build up from there. I will assume that you are using the UNIX command line and gcc as your environment for these examples, but if you are not all of the code will work fine. You will simply need to understand and use whatever compiler you have available.

Let's get started!

The Simplest C Program

Let's start with the simplest possible C program and use it both to understand the basics of C and the C compilation process. Type the following program into a standard text editor (vi or emacs on UNIX, Notepad on Windows or TeachText on a Macintosh). Then save the program to a file named samp.c. If you leave off .c, you will probably get some sort of error when you compile it, so make sure you remember the .c. Also, make sure that your editor does not automatically append some extra characters (like ".txt") to the name of the file. Here's the first program:

#include <stdio.h> 
 
int main()  
{    
    printf("This is output from my first program!\n");  
    return 0;
} 

When executed, this program will instruct the computer to print out the line "This is output from my first program!". Then the program quits. You can't get much simpler than that!

To compile this code take the following steps:

• On a UNIX machine, type gcc samp.c -o samp (if gcc does not work, try cc). This line invokes the C compiler called gcc, asks it to compile samp.c and asks it to place the executable file it creates under the name samp. To run the program, type samp (or, on some UNIX machines, ./samp).
• On a DOS or Windows machine using DJGPP, at an MS-DOS prompt type gcc samp.c -o samp.exe . This line invokes the C compiler called gcc, asks it to compile samp.c and asks it to place the executable file it creates under the name samp.exe. To run the program, type samp.
• If you are working with some other compiler or development system, read and follow the directions for the compiler you are using to compile and execute the program.

You should see the output "This is output from my first program!" when you run the program. Here is what happened when you compiled the program:

If you mistype the program, it either will not compile or it will not run. If the program does not compile or does not run correctly, edit it again and see where you went wrong in your typing. Fix the error and try again.

Let's walk through this program and start to see what the different lines are doing:

• This C program starts with #include <stdio.h> . This line includes the "standard I/O library" into your program. The standard I/O library lets you read input from the keyboard (called "standard in"), write output to the screen (called "standard out"), process text files stored on the disk, and so on. It is an extremely useful library. C has a large number of standard libraries like stdio, including string, time and math libraries. A library is simply a package of code that someone else has written to make your life easier (this page discusses libraries in detail).
• The line int main() declares the main function. Every C program must have a function named main somewhere in the code. We will learn more about functions shortly. At run time, program execution starts at the first line of the main function.
• In C, the { and } symbols mark the beinning and end of a block of code. In this case the block of code making up the main function contains 2 lines.
• The printf statement in C allows you to send output to standard out (for us, the screen). The portion in quotes is called the format string and describes how the data is to be formatted when printed. The format string can contain string literals such as "This is output from my first program!", symbols for carriage returns ("\n"), and operators as placeholders for variables (not shown in this example - see the next example). If you are using UNIX, you can type "man 3 printf" to get complete documentation for the printf function. If not, see the documentation included with your compiler for details about the printf function.
• The return 0; line causes the function to return an error code of 0 (no error) to the shell that started execution. More on this capability a bit later.

Here is another program that will help you learn more about printf:

#include <stdio.h> 
 
int main()  
{  
    int a, b, c;
    a = 5;
    b = 7;
    c = a + b;  
    printf("%d + %d = %d\n", a, b, c);  
    return 0;
} 

    int b;

    b = 5;

    printf("%d", b);

Type this program into a file and save it as "add.c". Compile it with the line gcc add.c -o add and then run it by typing add (or "./add"). You will see the line "5 + 7 = 12" as output.

Here is an explanation of the different lines in this program:

• The line int a, b, c; declares three integer variables named a, b and c. Integer variables hold whole numbers.
• The next line initializes the variable named a to the value 5.
• The next line sets b to 7.
• The next line adds a and b and "assigns" the result to c. The computer adds the value in a (5) to the value in b (7) to form the result 12 and then places that new value (12) into the variable c. The variable c is assigned the value 12. For this reason, the "=" in this line is called "the assignment operator".
• The printf statement then prints the line, "5 + 7 = 12". The "%d" placeholders in the printf statement act as placeholders for values. There are 3 "%d" placeholders, and then at the end of the printf line there are the 3 variable names: a, b and c. C matches up the first %d with a and substitutes 5 there. It matches the second %d with b and substitutes 7. It matches the third %d with c and substitutes 12. Then it prints the completed line to the screen: "5 + 7 = 12", The "+", the "=" and the spacing are a part of the format line and get embedded automatically between the %d operators as specified by the programmer.

This program is good, but it would be better if it read in the values 5 and 7 from the user instead of using constants. Try this program instead:

#include <stdio.h> 
 
int main()  
{  
    int a, b, c;
    printf("Enter the first value:");  
    scanf("%d", &a);     
    printf("Enter the second value:");  
    scanf("%d", &b);     
    c = a + b;  
    printf("%d + %d = %d\n", a, b, c);  
    return 0;
} 

Here's how this program works when you execute it:

Make the changes, then compile and run the program to make sure it works. Note that scanf uses the same sort of format string as printf (type man scanf for more info). Also note the & sign in front of a and b. This is the address operator in C: It returns the address of the variable, and it will not make sense until we discuss pointers. You must use the & operator in scanf on any variable of type char, int, or float, as well as structure types (which we will get to shortly). If you leave out the & operator, you will get an error when you run the program. Try it so that you can see what that sort of run-time error looks like.

    printf("Hello");

    printf("Hello\n");

    printf("%d", b);

    printf("The temperature is ");
    printf("%d", b);
    printf(" degrees\n", b);

    printf("The temperature is %d degrees\n", b);

    printf("%d + %d = %d\n", a, b, c);  

    scanf("%d", &b);

From this discussion you can see that it is very easy to create simple C programs. The printf and scanf functions will take a bit of practice to be completely understood, but once mastered they are extremely useful.

Try This!

• Modify the last example so it accepts three values instead of two and adds all three together.
• Try deleting or adding random characters or words in one of the programs and watch how the compiler reacts to these errors. For example, delete the b variable in the first line of the previous program and see what the compiler does when you forget to declare a variable. Delete a semicolon and see what happens. Leave out one of the braces. Remove one of the parentheses next to the main function. Make each error by itself and then run the program through the compiler to see what happens. By simulating errors like these you can learn about different compiler errors, and that will make your typos easier to find when you make them for real.

C errors to avoid

1. Using the wrong character case. Case matters in C, so you cannot type "Printf" or "PRINTF". It must be "printf".
2. Forgetting to use the & in scanf.
3. Too many or too few parameters following the format statement in printf or scanf.
4. Forgetting to declare a variable name before using it.

Branching and Looping in C

In C, both if statements and while loops rely on the idea of Boolean expressions. Here is a simple C program demonstrating an if statement:

#include <stdio.h> 
 
int main()  
{  
    int b;
    printf("Enter a value:");  
    scanf("%d", &b);     
    if (b < 0)    
        printf("The value is negative\n");
    return 0;
} 

This program accepts a number from the user. It then tests the number using an if statement to see if it is less than 0. If it is, the program prints a message. Otherwise, the program is silent. The (b < 0) portion of the program is the Boolean expression. C evaluates this expression to decide whether or not to print the message. If the Boolean expression evaluates to True, then C executes the single line immediately following the if statement (or a block of lines within braces immediately following the if statement). If the Boolean expression is False, then C skips the line or block of lines immediately following the if statement.

Here's another slightly more complex example:

#include <stdio.h> 
 
int main()  
{  
    int b;
    printf("Enter a value:");  
    scanf("%d", &b);     
    if (b < 0)    
        printf("The value is negative\n");
    else if (b == 0)    
        printf("The value is zero\n");
    else 
        printf("The value is positive\n");
    return 0;
} 

In this example, else if and else sections evaluate for zero and positive values as well.

Here is a more complicated Boolean expression:

if ((x==y) && (j>k))    
    z=1;  
else    
    q=10; 

This statment says, "if the value in variable x equals the value in variable y, and if the value in variable j is greater than the value in variable k, then set the variable z to 1, otherwise set the variable q to 10." You will use if statements like this throughout your C programs to make decisions. In general, most of the decisions you make will be simple ones like the first example, but on occasion things get more complicated.

Notice that C uses == to test for equality, while it uses = to assign a value to a variable. The && in C represents a Boolean AND operation. The following chart shows all boolean operators in C:

  equality          ==
  less than         <
  Greater than      >
  <=                <=
  >=                >=
  inequality        !=
  and               &&
  or                ||
  not               ! 

#include <stdio.h> 
 
int main()  
{    
    int a; 
    
    printf("Enter a number:");    
    scanf("%d", &a);    
    if (a)    
    {      
        printf("The value is True\n");   
    }  
    return 0;
} 

While statements are just as easy to use. For example:

while (a < b)  
{    
    printf("%d\n", a);
    a = a + 1;  
} 

Causes the two lines within the braces to be executed repeatedly until a is greater than or equal to b. The while statement in general works like this:

C also provides a "do-while" structure:

do  
{    
    printf("%d\n", a);
    a = a + 1;  
}  
while (a < b); 

The for loop in C is simply a shorthand way of expressing a while statement. For example, suppose you have the following code in C:

x=1;  
while (x<10)  
{    
    blah blah blah    
    x++; /* x++ is the same as saying x=x+1 */  
} 

You can convert this into a for loop as follows:

for(x=1; x<10; x++)  
{    
    blah blah blah  
} 

Note that the while loop contains an initialization step (x=1), a test step (x<10), and an increment step (x++). The for loop lets you put all three parts onto one line, but you can put anything into those three parts. For example, suppose you have the following loop:

a=1;  
b=6;  
while (a < b)  
{    
    a++;    
    printf("%d\n",a);  
} 

You can place this into a for statement as well:

for (a=1,b=6; a < b; a++,printf("%d\n",a)); 

It is slightly confusing, but it is possible. The comma operator lets you separate several different statements in the initialization and increment sections of the for loop (but not in the test section). Many C programmers like to pack a lot of information into a single line of C code. Many people think it makes the code harder to understand, so they break it up.

A Real Example!

Let's say that you would like to create a program that prints a Fahrenheit to Celsius conversion table. This is easily accomplished with a for loop or a while loop:

#include <stdio.h> 
 
int main()  
{  
    int a;
    a = 0;
    while (a <= 100)
    {
        printf("%4d degrees F = %4d degrees C\n",
            a, (a - 32) * 5 / 9);
        a = a + 10;  
    }
    return 0;
} 

If you run this program if will produce a table of values starting at 0 degrees F and ending at 100 degrees F. The output will look like this:

   0 degrees F =  -17 degrees C
  10 degrees F =  -12 degrees C
  20 degrees F =   -6 degrees C
  30 degrees F =   -1 degrees C
  40 degrees F =    4 degrees C
  50 degrees F =   10 degrees C
  60 degrees F =   15 degrees C
  70 degrees F =   21 degrees C
  80 degrees F =   26 degrees C
  90 degrees F =   32 degrees C
 100 degrees F =   37 degrees C

The table's values are in increments of 10 degrees. You can see that you can easily change the starting, ending or increment values of the table the program produces.

If you wanted your values to be more accurate, you could use floating point values instead:

#include <stdio.h> 
 
int main()  
{  
    float a;
    a = 0;
    while (a <= 100)
    {
        printf("%6.2f degrees F = %6.2f degrees C\n",
            a, (a - 32.0) * 5.0 / 9.0);
        a = a + 10;  
    }
    return 0;
} 

You can see that the declaration for a has been changed to a float and the %f symbol replaces the %d symbol in the printf statement. In addition, the %f symbol has some formatting applied to it: the value will be printed with 6 digits preceeding the decimal point and 2 digits following the decimal point.

Now let's say that we wanted to modify the program so that the temperature 98.6 is inserted in the table at the proper position. That is, we want the table to increment every 10 degrees, but we also want the table to enclude an extra line for 98.6 degrees F because that is the normal body temperature for a human being. The following program accomplishes the goal:

#include <stdio.h> 
 
int main()  
{  
    float a;
    a = 0;
    while (a <= 100)
    {
	if (a > 98.6)
        {
            printf("%6.2f degrees F = %6.2f degrees C\n",
                98.6, (98.6 - 32.0) * 5.0 / 9.0);
        }
        printf("%6.2f degrees F = %6.2f degrees C\n",
            a, (a - 32.0) * 5.0 / 9.0);
        a = a + 10;  
    }
    return 0;
} 

This program works if the ending value is 100, but if you change the ending value to 200 you will find that the program has a bug. It prints the line for 98.6 degrees too many times. We can fix that problem in several different ways. Here is one way:

#include <stdio.h> 
 
int main()  
{  
    float a, b;
    a = 0;
    b = -1;
    while (a <= 100)
    {
	if ((a > 98.6) && (b < 98.6))
        {
            printf("%6.2f degrees F = %6.2f degrees C\n",
                98.6, (98.6 - 32.0) * 5.0 / 9.0);
        }
        printf("%6.2f degrees F = %6.2f degrees C\n",
            a, (a - 32.0) * 5.0 / 9.0);
        b = a;
        a = a + 10;  
    }
    return 0;
} 

Try This!

1. Try changing the Fahrenheit to Celcius program so that it uses scanf to accept the starting, ending and increment value for the table from the user.
2. Add a heading line to the table that is produced.
3. Try to find a different solution to the bug fixed by the previous example.
4. Create a table that converts pounds to kilograms or miles to kilometers.

C Errors to avoid

1. Putting = when you mean == in an if or while statement.
2. Forgetting to increment the counter inside the while loop. If you forget to increment the counter you get an infinite loop - the loop never ends.
3. Accidentally putting a ; at the end of a for loop or if statement, so that the statement has no effect. For example,

   for (x=1; x<10; x++);    
       printf("%d\n",x); 
   

   only prints out one value because the semicolon after the for statement acts as the one line the for loop executes.