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:

**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; bool isPrimeNumber(int num); int main() { int num; cout << "Enter a positive number\n"; cin >> num; if(isPrimeNumber(num)) cout << num << " is a Prime Number"; else cout << num << " is NOT a Prime Number"; return 0; } bool isPrimeNumber(int num){ 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.

**Recommended Posts**