Once a function is written, you can call it any number of times. By calling a function, you transfer execution of the program to the function- definition code, which runs until it is finished or until it encounters a **return **statement.

**Example of Avg Function**

void main(){

double a=5;

double b=6;

cout<<"AVG is" << avg(a,b);

cout<<endl

}

double avg(double x, double y){

double v=(x+y)/2;

return v;

}

Note that only **main** is guaranteed to be executed. Other functions run only as called. But there are many ways a function can be called. For example, main can call a function A, which in turn calls B and C, which in turn calls D.

**Some Recommendation for function**

1. At the beginning of program , Declare the function

2. Then define the function.

3. Then other function call the function.

Like above program, first declare the function.

RETURN_TYPE Function_Name(Argument_List)

Then Define the Function

RETURN_TYPE Function_Name(Argument_List) {

Logical Statement;

}

Call the Function

n=avg(5,3);

**Now We can create for prime number function in c++**

//Declare function

bool prime(int n);

int main() {

int i;

while(TRUE)

{

cout<<"Enter Number (0-exit) and Press ENTER: "; cin>>i;

if(i==0)

BREAK;

if(prime(i))

{

cout<<i<<"is Prime Number"<<endl;

}

else

{

cout<<i<<"is not Prime Number"<<endl;

}

return 0;

}

}

bool prime(int n)

{

int i;

for (i=2;i<SQRT(n); i++)

{

if(n%i==0)

return false;

}

return true;

}