C++ Program to check whether a Number is Prime or not

August 23, 2021

To check if a number is prime or not in C++

Here , in this section we will discuss a program to check if a number is prime or not in C++. A number is said to be Prime Number if and only if it is divisible by one and itself.

Sample test case 1 :
Input : 12
Output :The given number is not a prime number.
Explanation : As 12 is divisible by 1, 2, 3, 4, 6 and 12 so, it is not a prime number.
Sample test case 2 :
Input : 23
Output :The given number is a prime number.
Explanation : 23 is divisible by 1 and 23, so it is a prime number.

Algorithm :

Take the number from the user and store it in variable n.
Initialize the variable flag with value 0.
Run a loop from 2 to sqrt(n)
Check for every value if n is divisible by any number then set flag =1 and break the loop.
If flag = 0 then print the given number is prime.
Else print the number is not prime.

C++ Program based on above algorithm

#include<bits/stdc++.h>
using namespace std;

int main () 
{
  int n, flag = 0;
  
  cout << "Enter the number to check if it is prime or not: ";
  cin >> n;
  
  for (int i = 2; i <= sqrt (n); i++)
    {
      if (n % i == 0){
	    flag = 1;
	    break;
	  }
    }
  
  if (flag == 0)
    cout << "The given number is prime";
  else 
   cout << "The given number is not prime";
  
  return 0;
}

Output :

Enter the number to check if it is prime or not: 23

The given number is prime