A prime is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 5 is prime but 10 is not prime. In this article, we will learn how to write a C++ program to check whether a number is prime or not.

Algorithm to Check Prime Numbers in C++
- Run a loop from 2 to n/2 using a variable i.
- Inside the loop, check whether n is divisible by i using the modulo operator ( n % i == 0 ).
- If n is not divisible by i, it means n is a prime number.
- If n is divisible by i, it means n has divisors other than 1 and itself, and thus, it is not a prime number.
Prime Number Program in C++
Below is the C++ program to check if a number is a prime number or not.
C++
#include <bits/stdc++.h>
using namespace std;
bool isPrime( int n)
{
if (n <= 1)
return false ;
for ( int i = 2; i <= n / 2; i++)
if (n % i == 0)
return false ;
return true ;
}
int main()
{
isPrime(11) ? cout << " true\n" : cout << " false\n" ;
isPrime(15) ? cout << " true\n" : cout << " false\n" ;
return 0;
}
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Complexity Analysis
- Time Complexity: O(n)
- Auxiliary Space: O(1)
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