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Function in C++

Introduction
By now we have used many functions in the programs, but those functions are compiler defined and part of C++ compiler, e.g sqrt(), strlen(), pow() etc. In this chapter we will study user defined functions means we will define our functions.
Function:A function is a sub-program that does a part of the whole task. The main advantage of a function is its re-usability- means when a function is defined it can be called (used) as many times as we want. Do not confuse with function and loop, loop repeats a block of statement in one place of the program, but function is called from different locations of a big programs. In simple words a function is nothing but a group of statement, these get executed when the function is called.
Part of a function

Return type - a function has to return a value - int, float, char, double etc. If nothing required then "void" may be used
Function name - follows the rules of variable naming
Parameter/ arguments list with type - parameters are the values send to a function, ecah parameter must be declare with its type individually separated by comma.
Function body - A group of processing statements

Syntax of function declaration

return_type function_name ( data_type para1, data_type para2 ...){ Statement - 1; Statement - 2; Statement - 3; . . . return ( value/ variable); }

Now the place in the program where the function is written - there are two places (1) write before the "main" function; (2) write after the "main" fumction, if written after main function then the function prototype to be declared in the "main" function. Function prototype is an indication that there is a function would be called from this function ( where the prototype is defining) is defined below. We would be following first method ie functions would be defined betwen <header files> and "main()" function.
Let us start with an exampl
Q62.Write a program to accept two numbers then find sum and average using function.

Program-62

#include <iostream.h>

int sum(int x, int y){
	int z;
	z=x+y;
	return z;
}

void main(){
  int a, b, c, av;
  cout<<"Enter the No-1:";
  cin>>a;
  cout<<"Enter the no-2:";
  cin>>b;
  c=sum(a,b);
  cout<<"The sum="<<c;
  av=sum(a,b)/2.0;
  cout<<"Average ="<<av;
}

In the above program see that function sum() is defined between <header file> and "main()" function. The function has two parameters x & y both type int, the function returns the sum. Parameters that appear in function definition are called formal parameters and the parameters that appear in the function call ( in the main see sum(a,b)) are called actual parameters. See the re-usability of sum function while finding average.
Let us see how the program works-

Program execution will start from main functin as usual
Accepts two numbers in variable "a" nad "b"
When "c=sum(a,b)" statement executes the value of "a" will be copied to variable "x" of the function "sum()", then the value of variable "b" will be copied to the variable "y" of the function "sum()"; the program control will be transferred to the function, the function will execute, the value it generates in the variable "z" will be transfrred to the calling function i.e "main()" function by the return statement, which will be caught by the variable "c" in the main function, now the program control is in "main()" function, so it displays the result.
One thing remember that a program with functions will execute slower then the same program without functions

See the figure below-
function execution

.
Remember that the variables(arguments/ parameters) in the function definition are alltogether different from the variables in the "main" functions, therefore if we take the same variable name (a, b ) in both places there is no harm.