# C++ Program to Check Prime Number By Creating a Function

A prime number is a whole number that is greater than one and the only factors of a prime number should be one and itself.

Some of the first prime numbers are −

2, 3, 5, 7, 11, 13 ,17

A program to check if a number is prime or not using a function is as follows.

## Example

Live Demo

#include <iostream>
using namespace std;
void isPrime(int n) {
int i, flag = 0;
for(i=2; i<=n/2; ++i) {
if(n%i==0) {
flag=1;
break;
}
}
if (flag==0)
cout<<n<<" is a prime number"<<endl;
else
cout<<n<<" is not a prime number"<<endl;
}
int main() {
isPrime(17);
isPrime(20);
return 0;
}

## Output

17 is a prime number
20 is not a prime number

The function isPrime() is used to find out if a number is prime or not. There is a loop that runs from 2 to half of n, where n is the number to be determined. Each of the values of the loop divide n. If the remainder of this division is 0, that means n is divisible by a number, not one or itself. So, it is not a prime number and the flag is set to 1. Then break statement is used to exit the loop as shown below −

for(i=2; i<=n/2; ++i) {
if(n%i==0) {
flag=1;
break;
}
}

If the value of flag remained zero, then the number is a prime number and that is displayed. If the value of flag was changed to one, then the number is not a prime number and that is displayed.

if (flag==0)
cout<<n<<" is a prime number";
else
cout<<n<<" is not a prime number";

The function isPrime() is called from the main() function for the values 17 and 20. This is shown as follows.

isPrime(17);
isPrime(20);

Updated on: 24-Jun-2020

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