Functions
Before reading this tutorial you should have knowledge of pointers.
Functions are building blocks of the programs. They
make the programs more modular and easy to read and manage. All C++
programs must contain the function main( ). The execution of the
program starts from the function main( ). A C++ program can contain any
number of functions according to the needs. The general form of the
function is: -
return_type function_name(parameter list)
{
body of the function
}
The function of consists of two parts function header and function body. The function header is:-
return_type function_name(parameter list)
The return_type specifies the type of the data the
function returns. The return_type can be void which means function does
not return any data type. The function_name is the name of the
function. The name of the function should begin with the alphabet or
underscore. The parameter list consists of variables separated with
comma along with their data types. The parameter list could be empty
which means the function do not contain any parameters. The parameter
list should contain both data type and name of the variable. For
example,
int factorial(int n, float j)
is the function header of the function factorial.
The return type is of integer which means function should return data
of type integer. The parameter list contains two variables n and j of
type integer and float respectively. The body of the function performs
the computations.
Function Declaration
A function declaration is made by declaring the
return type of the function, name of the function and the data types of
the parameters of the function. A function declaration is same as
the declaration of the variable. The function declaration is always
terminated by the semicolon. A call to the function cannot be made
unless it is declared. The general form of the declaration is:-
return_type function_name(parameter list);
For example function declaration can be
int factorial(int n1,float j1);
The variables name need not be same as the variables of parameter list of the function. Another method can be
int factorial(int , float);
The variables in the function declaration can be optional but data types are necessary.
Function Arguments
The information is transferred to the function by
the means of arguments when a call to a function is made. Arguments
contain the actual value which is to be passed to the function when it
is called. The sequence of the arguments in the call of the function
should be same as the sequence of the parameters in the parameter list
of the declaration of the function. The data types of the arguments
should correspond with the data types of the parameters. When a
function call is made arguments replace the parameters of the function.
The Return Statement and Return values
A return statement is used to exit from the
function where it is. It returns the execution of the program to the
point where the function call was made. It returns a value to the
calling code. The general form of the return statement is:-
return expression;
The expression evaluates to a value which has type
same as the return type specified in the function declaration. For
example the statement,
return(n);
is the return statement of the factorial function.
The type of variable n should be integer as specified in the
declaration of the factorial function. If a function has return type as
void then return statement does not contain any expression. It is
written as:-
return;
The function with return type as void can ignore
the return statement. The closing braces at the end indicate the exit
of the function. Here is a program which illustrates the working of
functions.
#include<iostream>
using namespace std;
int factorial(int n);
int main ()
{
int n1,fact;
cout <<"Enter the number whose factorial has to be calculated" << endl;
cin >> n1;
fact=factorial(n1);
cout << "The factorial of " << n1 << " is : " << fact << endl;
return(0);
}
int factorial(int n)
{
int i=0,fact=1;
if(n<=1)
{
return(1);
}
else
{
for(i=1;i<=n;i++)
{
fact=fact*i;
}
return(fact);
}
}
The result of the program is:-
The function factorial calculates the factorial of
the number entered by the user. If the number is less than or equal to 1
then function returns 1 else it returns the factorial of the number.
The statement
int factorial(int n);
is a declaration of the function. The return type
is of integer. The parameter list consists of one data type which is
integer. The statement
cout <<"Enter the number whose factorial has to be calculated" << endl;
cin >> n1;
makes the user enter the number whose factorial is
to be calculated. The variable n1 stores the number entered by the
user. The user has entered number 5. The statement
fact=factorial(n1);
makes a call to the function. The variable n1 is
now argument to the function factorial. The argument is mapped to the
parameters in the parameter list of the function. The function header
is
int factorial(int n)
The body of the function contains two return
statements. If the value entered by the user is less than and equal to 1
then value 1 is returned else computed factorial is returned. The type
of the expression returned is integer.
Parameter passing mechanism
There are two parameter passing mechanisms for passing arguments to functions such as pass by value and pass by reference.
Pass by value
In pass be value mechanism copies of the arguments
are created and which are stored in the temporary locations of the
memory. The parameters are mapped to the copies of the arguments
created. The changes made to the parameter do not affect the arguments.
Pass by value mechanism provides security to the calling program. Here
is a program which illustrates the working of pass by value mechanism.
#include<iostream>
using namespace std;
int add(int n);
int main()
{
int number,result;
number=5;
cout << " The initial value of number : " << number << endl;
result=add(number);
cout << " The final value of number : " << number << endl;
cout << " The result is : " << result << endl;
return(0);
}
int add(int number)
{
number=number+100;
return(number);
}
The result of the program is:-
The value of the variable number before calling the
function is 5. The function call is made and function adds 100 to the
parameter number. When the function is returned the result contains the
added value. The final value of the number remains same as 5. This
shows that operation on parameter does not produce effect on arguments.
Pass by reference
Pass by reference is the second way of passing
parameters to the function. The address of the argument is copied into
the parameter. The changes made to the parameter affect the arguments.
The address of the argument is passed to the function and function
modifies the values of the arguments in the calling function. Here is a
program which illustrates the working of pass by reference mechanism.
#include<iostream>
using namespace std;
int add(int &number);
int main ()
{
int number;
int result;
number=5;
cout << "The value of the variable number before calling the function : " << number << endl;
result=add(&number);
cout << "The value of the variable number after the function is returned : " << number << endl;
cout << "The value of result : " << result << endl;
return(0);
}
int add(int &p)
{
*p=*p+100;
return(*p);
}
The result of the program is:-
The address of the variable is passed to the
function. The variable p points to the memory address of the variable
number. The value is incremented by 100. It changes the actual contents
of the variable number. The value of variable number before calling
the function is 100 and after the function is returned the value of
variable number is changed to 105.
After an introduction of functions, let us move on to discuss structures.
