check prime no for large value in cpp
#include <iostream>
using namespace std;
int main()
{
int n, i;
bool isPrime = true;
cout << "Enter a positive integer: ";
cin >> n;
for(i = 2; i <= n / 2; ++i)
{
if(n % i == 0)
{
isPrime = false;
break;
}
}
if (isPrime)
cout << "This is a prime number";
else
cout << "This is not a prime number";
return 0;
}
// A optimized school method based C++ program to check
// if a number is prime
#include <bits/stdc++.h>
using namespace std;
bool isPrime(int n)
{
// Corner cases
if (n <= 1)
return false;
if (n <= 3)
return true;
// This is checked so that we can skip
// middle five numbers in below loop
if (n % 2 == 0 || n % 3 == 0)
return false;
for (int i = 5; i * i <= n; i = i + 6)
if (n % i == 0 || n % (i + 2) == 0)
return false;
return true;
}
// Driver Program to test above function
int main()
{
isPrime(11) ? cout << " true\n" : cout << " false\n";
isPrime(15) ? cout << " true\n" : cout << " false\n";
return 0;
}