Finding prime numbers in specific interval

Question

Find prime numbers in specific interval in specific amount of test cases.

Example is below: Input:

2
1 10
3 5

Output:

Notice the little space between the answer also.

here's my code:

#include <iostream>
#include <cmath>

void prime (int x, int y);

using namespace std;

int main()
{
    int t, x[10], y[10];
    cin >> t;

    for (int i = 0; i < t; i++)
        //for (int j = 0; j < t; j++)
        cin >> x[i] >> y[i];

    while (t > 0){
        for (int i = 0; i < t; i++)
            prime(x[i], y[i]);
        t--;
    }
}

void prime(int x, int y){
    bool prime = true;
    for (int i = x; i <= y; i++){
        for (int j = 2; j <= sqrt(i); j++){
            prime = true;
            if (i % j == 0)
                prime = false;
        }
        if (prime == true)
            cout << i << endl;
    }
    cout << endl;
}

Here's the output I get when I use the same input.

What am I doing wrong?

use [offset sieve of Eratothenes](http://stackoverflow.com/a/19641049/849891). C code [here](http://stackoverflow.com/a/9557173/849891). — Will Ness, Nov 21 '13 at 16:48

score 1 · Answer 1 · answered Nov 20 '13 at 19:13

1

You should move prime = true outside of the for loop. You are currently resetting it at every iteration. As far as the printing, you don't need that << endl when you print each line. You only need a space.

answered Nov 20 '13 at 19:13

quazzieclodo

831
4
10

I moved it in the first for() loop and it seems to fix the problem with the primes, but it still executes 3 times for some reason, the first interval is calculated twice, any reason for that? – user3002211 Nov 20 '13 at 19:16

score -1 · Accepted Answer · answered Nov 20 '13 at 19:17

Since no one's pointed it out yet, if you're wondering why you're getting three sets of output instead of two...

while (t > 0){
  for (int i = 0; i < t; i++)
    prime(x[i], y[i]);
  t--;
}

might be better phrased as

for (int i = 0; i < t; i++)
  prime(x[i], y[i]);

(The outer loop is what gives you extra output.)

Finding prime numbers in specific interval

2 Answers2