Functions in C: Structure of a Function, Declaration and Definition of a Function

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CS201 Introduction to Programming

Lecture Handout

Introduction to Programming

Lecture No. 9

Reading Material

Deitel & Deitel C++ How to Program

chapter 2

3.1, 3.2, 3.3, 3.4, 3.5,

3.6

Summary

Introduction

Functions

Structure of a Function

Declaration and Definition of a Function

Sample Program 1

Sample Program 2

Sample Program 3

Summary

Tips

Introduction

Now our toolkit is almost complete. The basic constructs of programming are sequence,

decision making and loops. You have learnt all these techniques. Now we can write

almost all kinds of programs. There are more techniques to further refine the programs.

One of the major programming constructs is Functions. C is a function-oriented language.

Every program is written in different functions.

In our daily life, we divide our tasks into sub tasks. Consider the making of a laboratory

stool.

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It has a seat and three legs. Now we need to make a seat and three legs out of wood. The

major task is to make a stool. Sub tasks are, make a seat and then fabricate three legs. The

legs should be identical. We can fashion one leg and then re-using this prototype, we

have to build two more identical legs. The last task is to assemble all these to make a

stool. We have a slightly difficult task and have broken down it into simpler pieces. This

is the concept of functional design or top-down designing. In top design, we look at the

problem from top i.e. identification of the problem. What we have to solve? Then refine it

and divide it into smaller pieces. We refine it again and divide it into smaller pieces. We

keep on doing it as long as we get easily manageable task. Let's consider an example like

home construction. From the top level, we have to construct a home. Then we say that we

need design of the home according to which the building will be constructed. We need to

construct rooms. How can we construct a room? We need bricks, cement, doors,

windows etc. Procurement of all of these things is tasks. Once we come down to the level

where a task is easily manageable and doable, we stop doing further refinement. When

we break up a task into smaller sub tasks, we stop at a reasonable level. Top-down

designing mechanism is based on the principle of 'divide and conquer' i.e. we divide a big

task into smaller tasks and then accomplish them.

Let's have a look at a simple example to understand the process of dividing big task into

simple ones. Suppose we want to know how many students are currently logged in the

LMS (Learning Management System) of VU. This task will be handed over to the

network administrator to find out the number of students currently logged in LMS of the

university. The network administrator will check the network activity or get this

information from the database and get the list of students currently logged in. The

number of students is counted from that list and the result is given back to us. What has

happened in this whole process? There was a simple request to find the number of

students currently logged in LMS. This request is delegated to the network administrator.

The network administrator performs this task and we get the result. In the mean time, we

can do some other task as we are not interested in the names or list of students. We only

want the number of students. This technique is known as parallel processing. In terms of

programming, network administrator has performed a function i.e. calculation of the

number of students. During this process, the network administrator also gets the list of

students which is hidden from us. So the information hiding is also a part of the function.

Some information is given to the network administrator (i.e. the request to calculate the

number of students currently logged in the LMS) while some information is provided

back to us (i.e. the number of students).

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Functions

The functions are like subtasks. They receive some information, do some process and

provide a result. Functions are invoked through a calling program. Calling program does

not need to know what the function is doing and how it is performing its task. There is a

specific function-calling methodology. The calling program calls a function by giving it

some information and receives the result.

We have a main ( ) in every C program. `main ( )' is also a function. When we write a

function, it must start with a name, parentheses, and surrounding braces just like with

main ( ). Functions are very important in code reusing.

There are two categories of functions:

1. Functions that return a value

2. Functions that do not return a value

Suppose, we have a function that calculates the square of an integer such that function

will return the square of the integer. Similarly we may have a function which displays

some information on the screen so this function is not supposed to return any value to the

calling program.

Structure of a Function

The declaration syntax of a function is as follows:

return-value-type function-name( argument-list )

{

declarations and statements

}

The first line is the function header and the declaration and statement part is the body of

the function.

return-value_type:

Function may or may not return a value. If a function returns a value, that must be of a

valid data type. This can only be one data type that means if a function returns an int data

type than it can only return int and not char or float. Return type may be int, float, char or

any other valid data type. How can we return some value from a function? The keyword

is return which is used to return some value from the function. It does two things, returns

some value to the calling program and also exits from the function. We can only return a

value (a variable or an expression which evaluates to some value) from a function. The

data type of the returning variable should match return_value_type data type.

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There may be some functions which do not return any value. For such functions, the

return_value_type is void. `void' is a keyword of `C' language. The default

return_value_type is of int data type i.e. if we do not mention any return_value_type with

a function, it will return an int value.

Function-name:

The same rules of variable naming conventions are applied to functions name. Function

name should be self-explanatory like square, squareRoot, circleArea etc.

argument-list:

Argument list contains the information which we pass to the function. Some function

does not need any information to perform the task. In this case, the argument list for such

functions will be empty. Arguments to a function are of valid data type like int number,

double radius etc.

Declarations and Statements:

This is the body of the function. It consists of declarations and statements. The task of the

function is performed in the body of the function.

Example:

//This function calculates the square of a number and returns it.

int square(int number)

{

int result = 0;

result = number * number;

return result;

}

Calling Mechanism:

How a program can use a function? It is very simple. The calling program just needs to

write the function name and provide its arguments (without data types). It is important to

note that while calling a function, we don't write the return value data type or the data

types of arguments.

Example:

//This program calculates the square of a given number

#include <iostream.h>

main()

{

int number, result;

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result = 0;

number = 0;

// Getting the input from the user

cout << " Please enter the number to calculate the square ";

cin >> number;

// Calling the function square(int number)

result = square(number);

cout << " The square of " << number << " is " << result;

}

Declaration and Definition of a Function

Declaration and definition are two different things. Declaration is the prototype of the

function, that includes the return type, name and argument list to the function and

definition is the actual function code. Declaration of a function is also known as signature

of a function.

As we declare a variable like int x; before using it in our program, similarly we need to

declare function before using it. Declaration and definition of a function can be combined

together if we write the complete function before the calling functions. Then we don't

need to declare it explicitly. If we have written all of our functions in a different file and

we call these functions from main( ) which is written in a different file. In this case, the

main( ) will not be compiled unless it knows about the functions declaration. Therefore

we write the declaration of functions before the main( ) function. Function declaration is

a one line statement in which we write the return type, name of the function and the data

type of arguments. Name of the arguments is not necessary. The definition of the function

contains the complete code of the function. It starts with the declaration statement with

the addition that in definition, we do write the names of the arguments. After this, we

write an opening brace and then all the statements, followed by a closing brace.

Example:

If the function square is defined in a separate file or after the calling function, then we

need to declare it:

Declaration:

int square ( int );

Definition:

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int square ( int number)

{

return (number * number ) ;

}

Here is the complete code of the program:

//This program calculates the square of a given number

#include <iostream.h>

// Function declarations.

int square(int);

main()

{

int number, result;

result = 0;

number = 0;

cout << " Please enter the number to calculate the square ";

cin >> number;

// Calling the function square(int number)

result = square(number);

cout << " The square of " << number << " is " << result;

}

// function to calculate the square of a number

int square ( int number)

{

return (number * number ) ;

}

A function in a calling program can take place as a stand-alone statement, on right- hand

side of a statement. This can be a part of an assignment expression.

Considering the above example, here are some more ways of function calling mechanism.

result = 10 + square (5);

result = square (number + 10);

result = square (number) + square (number + 1) + square (3 * number);

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cout << " The square of " << number << " is " << square (number);

In the above statements, we see that functions are used in assignment statements. In a

statement result = square(5); The square(5) function is called and the value which is

returned from that function (i.e. the value returned within the function using the return

keyword) is assigned to the variable result. In this case, the square(5) will return 25,

which will be assigned to variable result. There may be functions which do not return any

value. These functions can't be used in assignment statements. These functions are

written as stand-alone statements.

Sample Program 1

C is called function-oriented language. It is a very small language but there are lots of

functions in it. Function can be on a single line, a page or as complex as we want.

Problem statement:

Calculate the integer power of some number (xn).

Solution:

We want to get the power of some number. There is no operator for power function in C.

We need to write a function to calculate the power of x to n (i.e. xn). How can we

calculate the power of some number? To get the power of some number x to n, we need

to multiply x with x up to n times. Now what will be the input (arguments) to the

function? A number and power, as number can be a real number so we have to declare

number as a double date type and the power is an integer value so we will declare the

power as an integer. The power is an integer value so we will declare power as an integer.

The result will also be a real number so the return value type will be of double data type.

The function name should be descriptive, we can name this function as raiseToPow. The

declaration of the function is:

double raiseToPow ( double x, int power ) ;

To calculate the power of x up to power times, we need a loop which will be executed

power times. The definition of function is:

// function to calculate the power of some number

double raiseToPow ( double x , int power )

{

double result ;

int i ;

result = 1.0 ;

for ( i = 1 ; i <= power ; i ++ )

{

result *= x ; // same as result = result * x

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}

return ( result ) ;

}

Here is the program which is calling the above function.

// This program is calling a function raiseToPow.

#include <iostream.h>

//Function declaration

double raiseToPow ( double , int )

main ( )

{

double x ;

int i ;

cout << " Please enter the number " ;

cin >> x ;

cout << " Please enter the integer power that you want this number raised to " ;

cin >> i ;

cout << x << " raise to power " << i << " is equal to " << raiseToPow ( x , i ) ;

}

Now we have to consider what will happen to the values of arguments that are passed to

the function? As in the above program, we are passing x and i to the raiseToPow

function. Actually nothing is happening to the values of x and i. These values are

unchanged. A copy of values x and i are passed to the function and the values in the

calling program are unchanged. Such function calls are known as 'call by value'. There is

another way to call a function in which the function can change the values of variables

that are passed as arguments, of calling program. Such function call is known as call by

reference.

Sample Program 2

Problem statement:

Calculate the area of a ring.

Solution:

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We know that a ring consists of a small circle and a big circle. To calculate the area of a

ring, we have to subtract the area of small circle from the area of big circle. Area of any

circle is calculated as Pi * r2. We write a function to calculate the area of a circle and use

this function to calculate the area of small circle and big circle.

Following is the code of the function circleArea:

// Definition of the circleArea function.

double circleArea ( double radius )

{

// the value of Pi = 3.1415926

return ( 3.1415926 * radius * radius ) ;

}

Here is the complete code of the calling program.

// This program calculates the area of a ring

#include <iostream.h>

// function declaration.

double circleArea ( double);

void main ( )

{

double rad1 ;

double rad2 ;

double ringArea ;

cout << " Please enter the outer radius value: " ;

cin >> rad1 ;

cout << " Please enter the radius of the inner circle: " ;

cin >> rad2 ;

ringArea = circleArea ( rad1 ) circleArea (rad2 ) ;

cout<< " Area of the ring having inner raduis " << rad2 << " and the outer radius " <<

rad1 << " is " << ringArea ;

}

double circleArea ( double radius )

{

// the value of Pi = 3.1415926

return ( 3.1415926 * radius * radius ) ;

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}

Sample Program 3

There are some other kinds of functions which are used to test some condition. Such

functions return true or false. These functions are very important and used a lot in

programming. In C condition statements, the value zero (0) is considered as false and any

value other than zero is considered as true. So the return type of such functions is int. We

usually return 1 when we want the function to return true and return 0 when we want the

function to return 0. Here is a sample program to elaborate this.

Problem statement:

Write a function which tests that a given number is even or not? It should return true if

the number is even, otherwise return false.

Solution:

We already know the method of deciding whether a number is even or not. The name of

the function is isEven. Its return type will be int. It will take an int as an argument. So the

declaration of the function should be as below;

int isEven ( int ) ;

We can also use a function in the conditional statements like:

if ( isEven ( number ) )

If the number is even, the function will return none zero value (i.e. usually 1) and the if

statement will be evaluated as true. However, if the number is odd, the function will

return a zero value and the if statement is evaluated as false.

Here is a complete program.

// This program is calling a function to test the given number is even or not

#include <iostream.h>

// function declaration.

int isEven(int);

void main ( )

{

int number;

cout << " Please enter the number: " ;

cin >> number ;

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if ( isEven ( number ) )

{

cout << " The number entered is even " << endl;

}

else

{

cout << " The number entered is odd " << endl;

}

int isEven ( int number )

{

if ( 2 * ( number / 2 ) == number )

{

return 1;

}

else

{

return 0;

}

Summary

Functions are very good tools for code reuse. We have seen in the above example that the

area of two circles has been calculated without rewriting the code. This means that the

code has been reused. We can reuse the circleArea function to find the area of any circle.

A function performs a specific task. Functions also provide encapsulation. The calling

program does not know how the function is performing its task. So we can build up

modular form from small building blocks and build up more and more complex

programs.

If we are going to use a function in our program and the definition of the function is after

the calling program. The calling program needs to know how to call the function, what

the arguments are and what it will return. So its declaration must occur before usage. If

we do not declare a function before using, the compiler will give an error. If we define a

function before the calling program, then we do not need a separate declaration. The

function declaration is also known as function prototype or function signature. Whenever,

we need to build something, first of all we build a prototype of that thing and then later

on we build it. Similarly the function declaration is used as a prototype. We are following

the top- down methodology. We break the program into smaller modules and just declare

the functions and later on we can define these.

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Exercise:

1. Modify the raise to power function so that it can handle negative power of x, zero

and positive power of x.

2. Modify the area of ring function put in error checking mechanism.

Tips

We used functions for breaking complex problems into smaller pieces,

which is a top-down structured approach.

Each function should be a small module, self-contained. It should solve a

well defined problem.

Variable names and function names should be self- explanatory.

Always comment the code.

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