C++ loop each non-prime

Question

I'm trying to do this :

1. The User chooses one number
2. The program calls the isaPrime() function so we see if number is prime or not.

I wanted to do a loop so every time the number is not a prime, the user has to choose a new value.

Here's the code:

#include <iostream>
#include <math.h>

using namespace std;

bool isPrime (int num)
{
    if (num <=1)
        return false;
    else if (num == 2)
        return true;
    else if (num % 2 == 0)
        return false;
    else
    {
        bool prime = true;
        int divisor = 3;
        double num_d = static_cast<double>(num);
        int upperLimit = static_cast<int>(sqrt(num_d) +1);

        while (divisor <= upperLimit)
        {
            if (num % divisor == 0)
                prime = false;
            divisor +=2;
        }
        return prime;
    }
}

int main()
{
    int p;
    do {
      cout << "p : ";
      cin >> p;
      isPrime(p);
    } while (isPrime(p));

}

Answer 1

This should handle your user I/O loop. You just need to replace the isPrime function with your own implementation.

Code Listing

---

#include <iostream>
#include <iomanip>
#define isPrime(x) (1)

int main(void)
{
    using namespace std;

    const int maxchar = 5;
    string nationname;
    int input;
    int running = 1;

    while ( running )
    {
        cout << "Enter a number:";
        cin >> input;
        if ( isPrime(input ) )
        {
            // Do something
            running = 0;
        }
        else
        {
            cout << "Not a prime number. Please try again!" << endl;
        }
    }

    return 0;
}