using a user input in a loop and function

Question

My program takes a user input, int n, and prints out the first n amount of prime numbers. This is working as intended

eg. if user inputs 8 as n. the program will print :

2 3 5 7 11 13 17 19

My problem is adding the function isPrime(n) (which is not allowed to be changed)

here is what i've tried but im just getting the output :

2 3 5 7 11 13 17 19  0 is not a prime number,

when it should read 2 3 5 7 11 13 17 19 8 is not a prime number

#include "prime.h"
#include <iostream>

int main()
{


    int n;

    std::cout << "Enter a natural number: ";
    std::cin >> n;
        for (int i = 2; n > 0; ++i)
        {
            bool  Prime = true;
            for (int j = 2; j < i; ++j)
            {
                if (i  % j == 0)
                {
                    Prime = false;
                    break;
                }
            }
            if (Prime)
            {
                --n;
                std::cout << i << " ";

            }



        }

        if (isPrime(n))
        {
            std::cout << n << " is a prime number." << std::endl;
        }
        else
        {
            std::cout << n << " is not a prime number." << std::endl;
        }

        system("pause");

    }

prime.h :

#ifndef PRIME_H_RBH300111
#define PRIME_H_RBH300111

bool isPrime(int);

#endif
#pragma once

the definition of isPrime(int)

prime.cpp :

#include <cmath>

#include "prime.h"

bool isPrime(int n)
{
    if (n < 2)
    {
        return false;
    }
    else if (n == 2)
    {
        return true;
    }
    else if ((n % 2) == 0)
    {
        return false;
    }

}

I cannot alter the .h file of prime.cpp I just need the isPrime(n) function to work on the main() function code the user input n, does not seem to be taking the number 8. but instead 0 giving me the output. 0 is not a prime number rather than : n (8) is not a prime number

Are you sure you understand the instructions? I suspect you should be using isPrime() to test each number in the loop, instead of using your own loop that sets Prime, not doing isPrime(n). — Barmar
– Barmar, Commented May 18, 2017 at 20:27
@thesitg944 you just deleted a question you had about prime factorization code. I have a link here that provides explanation as to 4 coding ways of doing it, from un-optimized to extremely optimized. How it works, and mainly why you only need to seek sqrt of n in your for loop. — Edward
– Edward, Commented Jun 12, 2017 at 3:03

R Sahu · Accepted Answer · 2017-05-18 20:21:15Z

1

You are decrementing n in the loop. At the time the loop exits, the value of n is 0.

You can solve the problem by:

Using another variable to use in the loop.
Keeping a copy of the n and resetting the value of n after the loop exits.

Here's the second method:

int copyN = n;
for (int i = 2; n > 0; ++i)
{
   ...
}

n = copyN;

if (isPrime(n))
...

answered May 18, 2017 at 20:21

R Sahu

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Comments

Oblate · Accepted Answer · 2017-05-18 20:29:18Z

0

You are decrementing n in the for loop. The for loop has the condition 'n > 0', so you know n isn't > 0 when the loop finishes. You could either save the value of n in a different variable (i.e. "int nOrig = n;") and use that for the prime test, or use a different variable in the loop.

answered May 18, 2017 at 20:29

Oblate

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