A First C Program

Here is the traditional first C program:

#include <stdio.h>

int main(void)
{
    printf("Hello, world\n");
    return 0;
}

Once this program has been entered in a source file and translated into an executable file by the C compiler, we can run it. It will produce the following output, printed to the console window:

Hello, world

Let’s briefly examine the parts of this program:

#include <stdio.h> is an include directive which informs the C compiler that the program will use the built-in functions for input and output.
int main(void) introduces the main function, which is where the execution of the program begins. The curly braces enclose the statements which are part of the main function.
printf("Hello, world\n"); is a statement which prints the text Hello, world as output when the program is executed.
return 0; completes the main function by indicating that the program completed successfully. (Returning 0 traditionally means that the program worked correctly, while a non-zero value indicates that an error occurred.)

Variables and Assignment Statements

A variable is a small chunk of the computer’s memory used to store a value of a particular data type.

There are two things you can do with a variable:

Store a new value in the variable
Retrieve the value currently stored in the variable

In a C program, variables are created by using a variable declaration statement. A variable declaration has the following general form:

type identifier ;

“Type” refers to any C data type. A data type is a kind of data that a program can store and manipulate. Today, we will use the int data type. The int data type is used to represent integer quantities.

“Identifier” refers to the name of the variable. An identifier must follow certain rules:

It must begin with a letter or the underscore (“_”) character
The rest of the identifier must be comprised of letters, digits, or the underscore (“_”) character

For example, here are some examples of variable declarations:

int a;
int count;
int product;

An assignment statement stores a new value in a variable. Assignment statements have the following general form:

identifier = value ;

identifier is the name of the variable.

value is the new value of the variable. There are many ways to specify a value in C. For example, integer literals are values. The value of a variable can be obtained by specifying the name of the variable.

Examples:

// store the value 17 in the variable a
a = 17;

// store the value 0 in the variable product
product = 0;

// retrieve the current value of variable a and store it in the variable count
count = a;

Note that the “//” character sequence starts a comment. Comments are ignored by the compiler, but are useful for the programmer reading the program.

Printing Integer Values

Integer values may be printed to the console window using the printf function.

Example:

int answer;

answer = 42;

printf("The answer to life, the universe, and everything is %i\n", answer);

This code fragment prints the following text to the console window:

The answer to life, the universe, and everything is 42

The special character sequence %i means “get an integer value and print its decimal representation”. In this case, the value printed is the value stored in the variable answer, which is 42. %i is an example of a conversion specifier. Conversion specifiers are placeholders for values being printed.

Multiple occurrences of %i can be used with multiple values:

printf("%i, %i, %i...blast off!\n", 3, 2, 1);

This prints

3, 2, 1...blast off!

Reading Integer Values

The built-in scanf function reads a value or values from the keyboard. This allows the user of the program to enter information which can be processed by the program.

Here is the syntax of scanf in its most basic form:

scanf(format-string, & identifier);

The format-string is a specification of the kind of data the program wants to read from the keyboard. scanf format strings are very similar to the format strings used for printf. The main difference is that the conversion specifiers indicate data to be input from the user, rather than output to the console window.

The identifier is the name of a variable into which the information typed by the user should be stored.

Example:

int age;
printf("How old are you? ");
scanf("%i", &age);
printf("OK, you are %i years old\n", age);

In this example, the format string “%i” is used, meaning that the program wants an integer value from the keyboard. The value the user types will be stored in the age variable.

Here is possible output from the program when this code is executed (user input is in bold):

How old are you? 155
OK, you are 155 years old

The output above assumes that the user entered the value 155.