Prime Numbers

[mattswk]

Hi I'm new here. I'm trying to learn c++ on my own so I need all the help I can get! :) I'm having trouble figuring out what's wrong with my code. I'm trying to output the first N primes that the user inputs, but I get an error when N>1.
Thank you

// Prime Numbers

#include <iostream>

using namespace std;

int main(){


// Get input N from user and output first N prime numbers


int N;
int x = 2;
int counter = 0;
int m = 0;

cout << "Enter how many prime numbers you would like?\n";

cin >> N;

if (N == 1)
    cout << "\n" << "2\n";

else{
    for (x; x< 100000; x = x +1)
        for (m; m<10; m = m +1)
            if ((x%m==0)&&(x!=m))
                continue;
            else
                while (counter<N){
                    counter = counter +1;
                    cout << x << ", ";

    }

}


return 0;
}

[Jakee]

I did some trouble shooting and it seems to be in this area of the code:

            if ((x%m==0)&&(x!=m))
                continue;

If I set m to 1 it works but 0 it causes the program to crash.

[ne555]

> but I get an error
You need to describe your problem better.

Program received signal SIGFPE, Arithmetic exception.

performing a backtrace would indicate line 28 if ((x%m==0)&&(x!=m))

[AbstractionAnon]

Modulo is a form of division keeping the remainder.
What do you expect to happen when you divide by 0?

[mattswk]

Thanks for the replies. The error was simply that the program would crash so I wasn't sure where the problem was. I changed the code to int m = 1; and now the program runs, but now I just keep getting 2's. Now, I think I have something to work with so I will try now to see what I can do. Thanks, and if you have any further suggestions as to what to do, let me know!

[mattswk]

Btw, here is my code with some more comments and such

// Prime Numbers

#include <iostream>

using namespace std;

int main(){


// Get input N from user and output first N prime numbers


int N;
int x = 2;
int counter = 0;
int m = 1;

cout << "Enter how many prime numbers you would like?\n";

cin >> N;


for (x; x< 100000; x = x +1)       // Enumerate to 100000
        for (m; m<10; m = m +1)
            if ((x%m==0)&&(x!=m))        // Check x number from 1 to 9 to check if prime, ex: 4%2 == 0 is true an 4 != 0 so the number is not prime
                
                  continue;                // continue until a prime number is found
            else
                while (counter<N){            // Keep giving out prime numbers until counter reaches user N
                    counter = counter +1;
                    cout << x << ", ";

    }




return 0;
}

[AbstractionAnon]

Line 24: Why are you limiting divisors to < 10?

Lines 24,25: The usual for loop idiom is to initialize the starting value in the first term and to use the increment operator:
for (x=2; x< 100000; x++)

Line 29: Your while loop will keep putting out the same number until counter reaches N. What you really want to do here is to exit both for loops.

[Yanson]

The biggest problem that I see is that you are trying to do too much in your main function. Programming problems are better solved by breaking the code up into functions. write a function that takes a single integer and determines whether or not that integer is prime. then just have a for loop that feeds that function integers.

also there are many posts about prime numbers so you might want to look some of them up to get ideas.

[mattswk]

Thanks for the advice Yanson and Anon. Btw, what do you mean exit both for loops? Would I put a break; in the while loop? I feel like that would break out of the loop before all the prime numbers were outputted. If you coul clarify that a little more, I would appreciate it. Thanks

[AbstractionAnon]

When counter (number of primes found) reaches N (number of primes requested), what is the point of continuing? My point was that you whould exit the program at that point.

[mattswk]

Ok thanks it works now. Here is the code if anyone is interested.

#include <iostream >

using namespace std;

int main()
{
    int N;
    cin >> N;
    for(int i= 2; N>0; ++i)
    {
        bool isPrime = true ;
        for (int j= 2; j<i; ++j)
        {
            if(i %j== 0)
            {
                isPrime = false ;
                break ;
            }
        }
        if(isPrime)
        {
            --N;
            cout << i << "\n";
        }
    }
return 0;
}

[mattswk]

I think the key here was line 22 with the --N so I didn't have to mess with the while loop.