Programming in C and Data Structures Module 3
FUNCTIONS
A function as series of instructions or group of statements with one specific purpose.
A function is a program segment that carries out some specific, well defined task.
A function is a self contained block of code that performs a particular task.
4.1 Types of functions
C functions can be classified into two types,
1. Library functions /pre defined functions /standard functions /built in functions
2. User defined functions
1. Library functions /pre defined functions /standard functions/Built in Functions
These functions are defined in the library of C compiler which are used frequently in
the C program.
These functions are written by designers of c compiler.
C supports many built in functions like
Mathematical functions
String manipulation functions
Input and output functions
Memory management functions
Error handling functions
EXAMPLE:
pow(x,y)-computes xy
sqrt(x)-computes square root of x
printf()- used to print the data on the screen
scanf()-used to read the data from keyboard.
2. User Defined Functions
The functions written by the programmer /user to do the specific tasks are called user
defined function(UDF’s).
The user can construct their own functions to perform some specific task. This type of
functions created by the user is termed as User defined functions.
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Programming in C and Data Structures Module 3
4.2 Elements of User Defined Function
The Three Elements of User Defined function structure consists of :
1. Function Definition
2. Function Declaration
3. Function call
1. Function Definition:
A program Module written to achieve a specific task is called as function definition.
Each function definition consists of two parts:
i. Function header
ii. Function body
General syntax of function definition
Function Definition Syntax Function Definition Example
datatypefunctionname(parameters) void add()
{ {
declaration part; int sum,a,b;
executable part; printf(“enter a and b\n”);
return statement; scanf(“%d%d”,&a,&b);
sum=a+b;
printf(“sum is %d”,sum);
} }
i. Function header
Syntax
datatype functionname(parameters)
It consists of three parts
a) Datatype:
The data type can be int,float,char,double,void.
This is the data type of the value that the function is expected to return to
calling function.
b) functionname:
The name of the function.
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Programming in C and Data Structures Module 3
It should be a valid identifier.
c) parameters
The parameters are list of variables enclosed within parenthesis.
The list of variables should be separated by comma.
Ex: void add( int a, int b)
In the above example the return type of the function is void
the name of the function is add and
The parameters are 'a' and 'b' of type integer.
ii. Function body
The function body consists of the set of instructions enclosed between { and } .
The function body consists of following three elements:
a) declaration part: variables used in function body.
b) executable part: set of Statements or instructions to do specific activity.
c) return : It is a keyword,it is used to return control back to calling function.
If a function is not returning value then statement is:
return;
If a function is returning value then statement is:
return value;
2. Function Declaration
The process of declaring the function before they are used is called as function
declaration or function prototype.
function declaration Consists of the data type of function, name of the function
and parameter list ending with semicolon.
Function Declaration Syntax
datatypefunctionname(type p1,type p2,………type pn);
Example
int add(int a, int b);
void add(int a, int b);
Note: The function declaration should end with a semicolon ;
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Programming in C and Data Structures Module 3
3. Function Call:
The method of calling a function to achieve a specific task is called as function
call.
A function call is defined as function name followed by semicolon.
A function call is nothing but invoking a function at the required place in the
program to achieve a specific task.
Ex:
void main()
{
add( ); // function call without parameter
}
4.3 Formal Parameters and Actual Parameters
Formal Parameters:
The variables defined in the function header of function definition are called
formal parameters.
All the variables should be separately declared and each declaration must be
separated by commas.
The formal parameters receive the data from actual parameters.
Actual Parameters:
The variables that are used when a function is invoked)in function call) are called
actual parameters.
Using actual parameters, the data can be transferred from calling function.
to the called function.
The corresponding formal parameters in the function definition receive them.
The actual parameters and formal parameters must match in number and type of
data.
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Programming in C and Data Structures Module 3
Differences between Actual and Formal Parameters
Actual Parameters Formal Parameters
Actual parameters are also called as Formal parameters are also
argument list. called as dummy parameters.
Ex: add(m,n) Ex:int add(int a, int b)
The variables used in function call are The variables defined in function
called as actual parameters header are called formal
parameters
Actual parameters are used in calling Formal parameters are used in
function when a function is called or the function header of a called
invoked function.
Ex: add(m,n) Example:
Here, m and n are called actual int add(int a, int b)
parameters { ………..
}
Here, a and b are called formal
parameters.
Actual parameters sends data to the Formal parameters receive data
formal parameters from the actual parameters.
Example:
4.4 Categories of the functions
1. Function with no parameters and no return values
2. Function with no parameters and return values.
3. Function with parameters and no return values
4. Function with parameters and return values
1. Function with no parameters and no return values
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Programming in C and Data Structures Module 3
1. Function with no parameters and no return values
(void function without parameter)
Calling function Called function
/*program to find sum of two numbers
using function*/
#include<stdio.h> void add ( )
void add( ); {
void main( ) int sum;
{ printf(“enter a and b values\n”);
add( ); scanf(“%d%d”,&a,&b);
} sum=a+b;
printf(“\n The sum is %d”, sum);
return;
}
In this category no data is transferred from calling function to called function,
hence called function cannot receive any values.
In the above example,no arguments are passed to user defined function add( ).
Hence no parameter are defined in function header.
When the control is transferred from calling function to called function a ,and b
values are read,they are added,the result is printed on monitor.
When return statement is executed ,control is transferred from called function/add
to calling function/main.
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Programming in C and Data Structures Module 3
2. Function with parameters and no return values
(void function with parameter)
Calling function Called function
/*program to find sum of two numbers
using function*/
#include<stdio.h>
void add(int m, int n);
void main() void add(int a, int b)
{ {
int m,n; int sum;
printf(“enter values for m and n:”); sum = a+b;
scanf(“%d %d”,&m,&n); printf(“sum is:%d”,sum);
add(m,n); return;
} }
In this category, there is data transfer from the calling function to the called
function using parameters.
But there is no data transfer from called function to the calling function.
The values of actual parameters m and n are copied into formal parameters a and b.
The value of a and b are added and result stored in sum is displayed on the screen
in called function itself.
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Programming in C and Data Structures Module 3
3. Function with no parameters and with return values
Calling function Called function
/*program to find sum of two numbers
using function*/
#include<stdio.h> int add( ) /* function header */
int add();
{
void main()
{ int a,b,sum;
int result; printf(“enter values for a and
result=add( ); b:”);
printf(“sum is:%d”,result); scanf(“%d %d”,&a,&b);
} sum= a+b;
return sum;
}
In this category there is no data transfer from the calling function to the
called function.
But, there is data transfer from called function to the calling function.
No arguments are passed to the function add( ). So, no parameters are defined in
the function header
When the function returns a value, the calling function receives one value from
the called function and assigns to variable result.
The result value is printed in calling function.
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Programming in C and Data Structures Module 3
4. Function with parameters and with return values
Calling function Called function
/*program to find sum of two numbers
using function*/
#include<stdio.h>
int add();
int add(int a, int b) /* function header */
void main()
{
{
int result,m,n; int sum;
printf(“enter values for m and sum= a+b;
n:”); return sum;
scanf(“%d %d”,&m,&n); }
result=add(m,n);
printf(“sum is:%d”,result);
}
In this category, there is data transfer between the calling function and called
function.
When Actual parameters values are passed, the formal parameters in called
function can receive the values from the calling function.
When the add function returns a value, the calling function receives a value from
the called function.
The values of actual parameters m and n are copied into formal parameters a and
b.
Sum is computed and returned back to calling function which is assigned to
variable result.
4.5 Passing parameters to functions or Types of argument passing
The different ways of passing parameters to the function are:
Pass by value or Call by value
Pass by address or Call by address
1. Call by value:
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Programming in C and Data Structures Module 3
In call by value, the values of actual parameters are copied into formal parameters.
The formal parameters contain only a copy of the actual parameters.
So, even if the values of the formal parameters changes in the called function, the
values of the actual parameters are not changed.
The concept of call by value can be explained by considering the following program.
Example:
#include<stdio.h>
void swap(int a,int b);
void main()
{
int m,n;
printf("enter values for a and b:");
scanf("%d %d",&m,&n);
printf("the values before swapping are m=%d n=%d \n",m,n);
swap(m,n);
printf("the values after swapping are m=%d n=%d \n",m,n);
}
void swap(int a, int b)
{
int temp;
temp=a;
a=b;
b=temp;
}
Execution starts from function main( ) and we will read the values for variables
m and n, assume we are reading 10 and 20 respectively.
We will print the values before swapping it will print 10 and 20.
The function swap( ) is called with actual parameters m=10 and n=20.
In the function header of function swap( ), the formal parameters a and b
receive the values 10 and 20.
In the function swap( ), the values of a and b are exchanged.
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Programming in C and Data Structures Module 3
But, the values of actual parameters m and n in function main( ) have not been
exchanged.
The change is not reflected back to calling function.
2. Call by Address
In Call by Address, when a function is called, the addresses of actual
parameters are sent.
In the called function, the formal parameters should be declared as pointers
with the same type as the actual parameters.
The addresses of actual parameters are copied into formal parameters.
Using these addresses the values of the actual parameters can be changed.
This way of changing the actual parameters indirectly using the addresses of
actual parameters is known as pass by address.
Example:
#include<stdio.h>
void swap(int a,int b);
void main()
{
int m,n;
printf("enter values for a and b:");
scanf("%d %d",&m,&n);
printf("the values before swapping are m=%d n=%d \n",m,n);
swap(&m,&n);
printf("the values after swapping are m=%d n=%d \n",m,n);
}
void swap(int*a, int*b)
{
int temp;
temp=*a;
*a=*b;
*b=temp;
}
NOTE:
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Programming in C and Data Structures Module 3
Pointer: A pointer is a variable that is used to store the address of another variable.
Syntax: datatype *variablename;
Example: int *p;
#include<stdio.h>
void main()
{
inta ,*p;
p=&a;
}
In the above program p is a pointer variable, which is storing the address of variable a.
Differences between Call by Value and Call by reference
Call by Value Call by Address
When a function is called the values When a function is called the addresses of
of variables are passed variables are passed
The type of formal parameters should The type of formal parameters should be
be same as type of actual parameters same as type of actual parameters, but
they have to be declared as pointers.
Formal parameters contains the Formal parameters contain the addresses
values of actual parameters of actual parameters.
4.6 Scope and Life time of a variable
Scope of a variable is defined as the region or boundary of the program in which
the variable is visible. There are two types
(i) Global Scope
(ii) Local Scope
i. Global Scope:
The variables that are defined outside a block have global scope.
That is any variable defined in global area of a program is visible from its
definition until the end of the program.
For Example, the variables declared before all the functions are visible
everywhere in the program and they have global scope.
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Programming in C and Data Structures Module 3
ii. Local Scope
a. The variables that are defined inside a block have local scope.
b. They exist only from thepoint of their declaration until the end of the block.
c. They are not visible outside the block.
Life Span of a variable
The life span of a variable is defined as the period during which a variable is
active during execution of a program.
For Example
The life span of a global variable is the life span of the program.
The life span of local variables is the life span of the function, they are
created.
Storage Classes
There are following storage classes which can be used in a C Program:
i. Global variables
ii. Local variables
iii. Static variables
iv. Register variables
i. Global variables:
These are the variables which are defined before all functions in global area
of the program.
Memory is allocated only once to these variables and initialized to zero.
These variables can be accessed by any function and are alive and active
throughout the program.
Memory is deallocated when program execution is over.
ii. Local variables(automatic variables)
These are the variables which are defined within a functions.
These variables are also called as automatic variables.
The scope of these variables are limited only to the function in which they
are declared and cannot be accessed outside the function.
iii. Static variables
The variables that are declared using the keyword static are called static
variables.
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Programming in C and Data Structures Module 3
The static variables can be declared outside the function and inside the
function. They have the characteristics of both local and global variables.
Static can also be defined within a function.
Ex:
static int a,b;
iv. Register variables
Any variables declared with the qualifier register is called a register variable.
This declaration instructs the compiler that the variable under use is to be
stored in one of the registers but not in main memory.
Register access is much faster compared to memory access.
Ex:
register int a;
Recursion
Recursion is a method of solving the problem where the solution to a problem
depends on solutions to smaller instances of the same problem.
Recursive function is a function that calls itself during the execution.
The two types of recursion are
1.Direct Recursion
2. Indirect Recursion
1.Direct recursion
A recursive function that invokes itself is said to have direct recursion.
For example factorial function calls itself hence it is called direct recursion.
2. Indirect recursion
A function which contains call to another function which in turn contains calls
another function, and so on.
Design of recursive function
Any recursive function has two elements:
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Programming in C and Data Structures Module 3
i. Base case
ii. General case
i. Base case
The statement that solves the problem is called base case.
Every recursive function must have at least one base case.
It is a special case whose solution can be obtained without using recursion.
A base case serves two purposes:
i). It act as a terminating condition.
ii). the recursive function obtains the solution from the base case it
reaches.
ii. General case:
The statement that reduces the size of the problem is called general case.
This is done by calling the same function with reduced size.
In general recursive function of factorial problem can be written as
5!=5*4!
4!=4*3!
3!=3*2!
2!=2*1!
1!=1*0!
0!==1 This is a base case
1!=1*0!=1*1=1
2!=2*1!=2*1=2
3!=3*2!, 3!=3*2=6
4!=4*3! , 4!=4*6=24
5!=5*4!, 5*=24,
120(answer)
n!= 1 if n=0 // Base case
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Programming in C and Data Structures Module 3
n* (n-1)! Otherwise // General case
Write (n-1)! as fact(n-1)
n!= 1 if n=0 // Base case
n* fact(n-1) Otherwise // General case
Example 1.
/******* Factorial of a given number using Recursion ******/
#include<stdio.h>
int fact(int n);
void main( )
{
int num,result;
printf("enter number:");
scanf("%d",&num);
result=fact(num);
printf("The factorial of a number is: %d",result);
}
int fact(int n)
{
if(n==0)
return 1;
else
return (n*fact(n-1));
}
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