Errors in Prime Numbers Program (C++)

Question

Possible Duplicate:
C++ Prime Numbers program

I am working on a c++ program to calculate all prime numbers between 3 and an integer 'x'. When I enter 10 as 'x' I am getting the output: 3 5 5 5 7 7 7 7 7 9

Can anyone tell me how to fix this?

#include <iostream>
#include <cmath>

using std::cout;
using std::endl;
using std::cin;

int main(){

   int x;
   int i;
   int j;

   cout << "Please enter an integer 'x' greater than 3: " << endl;

   cin >> x;

   if (x <= 3){

        cout << "Please enter new value 'x' greater than 3: " << endl;

        cin >> x;
   }
        for(int i=3; i<=x; i++){
                for(j=2; j<i; j++){
                   if(i%j == 0)
                        break;
                   else if(i == j+1);
                        cout << i << endl;
                   }
        }
        return 0;
}

Ah, the old semi-colon on if-statement thing. – paddy Jan 29 '13 at 04:03 — paddy, Jan 29 '13 at 04:03

score 4 · Accepted Answer · answered Jan 29 '13 at 04:05

4

Offending line:

else if(i == j+1);

Remove the semi-colon.

answered Jan 29 '13 at 04:05

paddy

60,864
6
61
103

This made it work perfectly, thank you very much! :) – Jessica Jan 29 '13 at 04:08
No worries. For an extra challenge, you should try implementing the Sieve of Eratosthenes as suggested by Arun Saha. I did a little experiment with it a couple of days ago, and found it took less than 200 milliseconds to calculate primes up to 15 million. – paddy Jan 29 '13 at 04:24

score 1 · Answer 2 · answered Jan 29 '13 at 04:05

1

You may want to check out the Sieve of Eratosthenes.

answered Jan 29 '13 at 04:05

Arun

19,750
10
51
60

Good suggestion, but I would recommend mastering the basics first. – paddy Jan 29 '13 at 04:05
1

+1 for the most optimal (based on time complexity) solution provided.. – AurA Jan 29 '13 at 04:07

Errors in Prime Numbers Program (C++)

2 Answers2