# C++ Program to Check Prime Number Using Function

• Write a C++ program to check whether a number is prime number or not using function.

In this program, we will learn about prime numbers and how to check whether a number is prime number or not. Here s the formal definition of prime numbers:

A Prime number is a natural number greater than 1 that is only divisible by either 1 or itself. In other words, a prime is not divisible by any other number other than itself. All numbers other than prime numbers are known as composite numbers.

First few prime numbers are : 2 3 5 7 11 13 17 19 23 29 ...

## C++ program to check a prime number using function

```// C++ program to check prime number
#include <iostream>
using namespace std;

int main() {
int num;
cout << "Enter a positive number\n";
cin >> num;

cout << num << " is a Prime Number";
else
cout << num << " is NOT a Prime Number";

return 0;
}

bool isPrime = true;
int i;
// Check whether num is divisible by any number between 2 to (num/2)
for(i = 2; i <=(num/2); ++i) {
if(num%i==0) {
isPrime=false;
break;
}
}

return isPrime;
}
```
Output
```Enter a positive number
13
13 is a Prime Number
```
```Enter a positive number
15
15 is NOT a Prime Number
```

In this program, we first take an integer as input from user using cin and store it in a variable num. We then call isPrimeNumber function by passing num to check whether num is prime number or not.

Here we defined a function isPrimeNumber which check whether a number is prime number or not. If number is prime then it returns true otherwise false. To test whether a number is prime or not we are using brute force approach by testing whether num is a multiple of any integer between 2 and num/2. If num is divisible by any number between 2 and num/2 then num is not a prime number.

This is the most basic method of checking the primality of a given integer num and is called trial division.

Finally, based on the return value of isPrimeNumber function, we display message on screen saying whether number is prime number or not.