Functions (a)

In This Section:
Types of Functions, Built-in Functions, User-Defined Functions

Types of Functions

There are several different types of functions in C. So far, we have used one type of function: built-in C functions, like printf( ) and scanf( ). These functions are part of the C programming language.

Another type is called a user-defined function. This is a function which the programmer creates and uses in a C program. We will learn how to create these functions in this part of the tutorial.

We can also distinguish between functions which return values and those which don't -- like void main( ). Technically speaking, all functions return values, but in C we can choose to eliminate this process by using void. So far, we have not used our main( ) function to return any values, so we voided it and let the body functions, like printf( ) return values.

A function which return values produces some result -- like printing text on the screen, or scanning a variable, etc. Let's examine the different types of functions, so we can understand them better.

Built-in Functions

C has many built-in functions that you can use in your programs. So far we have learned 15 built-in functions:

If you can't remember how to use each of these functions, go back and review the previous sections of this tutorial.

User-Defined Functions

If you have a special set of instructions that aren't in a built-in function, you can create a user-defined function. Here are the steps:

1. give your function a name that isn't already used in C (by built-in functions, types of variables, keywords, etc.)
2. create a function header, which contains three things:

a. the type of variable (int, char, double, etc.) that the function will produce (return)

b. the name of the function, which can be one or more words (but put underscores _ or CapitalLetters connecting these words, because no spaces are allowed)

c. the parameters of the function, which are the names and types of variables inside your function

3. create a function body, which contains the operations to be completed when you call the function to run

Function Header

Here is an example of a function header:

This function expects an integer variable, so we begin with int.

Then we type a space followed by the name of the function, Square.

Next, we put the parameters in parentheses: (int num).

Function Body

Now we can add variables and functions to create a function body:

{
return (num * num);
}

This function calculates the square of num, which is num * num. This function returns the value of num * num.

Location and Call

To use this function, we must write it before void main( ) or after void main( ). We don't want to write it inside void main( ), since Square( ) is a function separate from main( ).

#include<stdio.h>
int Square(int num)
{
return (num * num);
}

Inside void main( ), we must call the function Square( ) to run. This means triggering the function to activate. This involves three things:

1. a variable equal to the value returned from the function
2. the name of the function
3. the variable passed to the function

Here is an example:

void main( )
{
int num;
int total;
total = Square(num);
}

The Square Function

Using main( ) Plus Two More Functions

You can create more than one function in a C program. Each function will need its own return statement and its own variable to store the returned value. Here is an example:

Let's analyze this program step-by-step: