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:
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.
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