C++ loop each non-prime

Question

I'm trying to do this :

The User chooses one number
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));

}

This is not really funny... If I came here it means I couldn't do it by myself. If you are just here for useless post like that just get out of there. — Amat Erasu, Aug 24 '15 at 21:42
http://stackoverflow.com/help/mcve Question is off-topic period! What do you go to do now? — πάντα ῥεῖ, Aug 24 '15 at 21:43
So, what's stopping you from editing your post and pasting the code, not a picture of the code? — Thomas Matthews, Aug 24 '15 at 21:45
_"I did it all these things..."_ You like to share with us not using off site links also?? — πάντα ῥεῖ, Aug 24 '15 at 21:46
Oh I understand now, okay sorry for this... I'll post the code the next time. — Amat Erasu, Aug 24 '15 at 21:50

score 2 · Accepted Answer · answered Aug 24 '15 at 21:44

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;
}

score 0 · Answer 2 · edited Aug 24 '15 at 21:53

0

If you delete the do loop your code will work. This is because your loop will run as long as the isprime returns true, and will stop running when isprime returns false.

Also if you want to display whether or not the number is prime you should use cout << isPrime(p);

thus your main funcion should look like

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

edited Aug 24 '15 at 21:53

πάντα ῥεῖ

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answered Aug 24 '15 at 21:45

Erk

41
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That's not what I wanted, Dogbert answered. But thanks for the contribution ! – Amat Erasu Aug 24 '15 at 21:55

score 0 · Answer 3 · answered Aug 24 '15 at 21:57

According to your question: When user chooses a non prime number the loop iterates. What you are doing is reverse so just replace

else{
bool prime =false   //replaced to true
//your code
while (divisor <= upperLimit)
    {
        if (num % divisor == 0)
            {
                prime = true;   //replaced to true 
                break;
            }
        divisor +=2;
    }
    return prime;

C++ loop each non-prime

3 Answers3