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Programming In C: Built In & User Defined 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:

main( )
printf( )
scanf( )
gets( )
puts( )
strcpy( )
strlen( )
strcmp( )
stricmp( )
strcat( )
strstr( )
isalpha( )
isdigit( )
isupper( )
islower( )

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

  1. 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:

int Square(int num)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. 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

#include<stdio.h>
int Square(int num)
{
return (num * num);
}
void main( )
{
int num;
int total;
printf(“Type a number: “);
scanf(“%d”, &num);
total = Square(num);
printf(“Your number squared is %d.”, total);
}

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:

#include<stdio.h>
int Thirteen(int born)
{
return (born + 13);
}
int Nineteen(int born)
{
return (born + 19);
}
void main( )
{
int born;
int teen1;
int teen2;
printf(“What year were you born? “);
scanf(“%d”, &born);
teen1 = Thirteen(born);
teen2 = Nineteen(born);
printf(“You were a teenager from %d to %d.”, teen1teen2);
}

 

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

#include<stdio.h>
int Thirteen(int born)
{
return (born + 13);
}
This is the first function:Thirteen, which uses the passed variable born. This function returns a value of born + 13.

int Nineteen(int born)
{
return (born + 19);
}
This is the second function:Nineteen, which uses the passed variable born. This function returns a value of born + 19.

void main( )
{
int born;
int teen1;
int teen2;
In the main( ) function, declare three variables:

  1. the passed variable
  2. the variable to store the returned value from the first function
  3. the variable to store the returned value from the second function

 


printf(“What year were you born? “);
scanf(“%d”, &born);
Ask the viewer to enter a value, then scanf it.

teen1 = Thirteen(born);
teen2 = Nineteen(born);
Pass the variable to each function, and store the returned value in our two variables.

printf(“You were a teenager from %d to %d.”, teen1,teen2);
}
Print the result.

About Michael Boguslavskiy

Michael Boguslavskiy is a full-stack developer & online presence consultant based out of New York City. He's been offering freelance marketing & development services for over a decade. He currently manages Rapid Purple - and online webmaster resources center; and Media Explode - a full service marketing agency.

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