prime pairs from 3 - 10,000

i found a set of beginner's problems to solve...i came up with a solution to this one:

print out all the prime pairs (prime numbers that differ only by 2) from 3 - 10,000.

Code:

---


#include <stdio.h>
#include <stdlib.h>

main(){
    int this_number, divisor, not_prime;
    int array[1500];
    int position = 0;
    this_number = 3;

    while(this_number < 10000){
            divisor = this_number / 2;
            not_prime = 0;
            while(divisor > 1){
                    if(this_number % divisor == 0){
                            not_prime = 1;
                            divisor = 0;
                    }
                    else
                      divisor = divisor-1;
            }
            if(not_prime == 0) {
                    array[position] = this_number;
                    position = position + 1;
            }
            this_number = this_number + 1;
    }
    position = 0;
    while (position < 1500) {
        if ((array[(position + 1)] - array[position]) == 2) {
            printf ("%d and %d are prime pairs.\n", array[position], array[(position + 1)]);
        }
        position = position + 1;
    }
    getchar();
    exit(EXIT_SUCCESS);
}

---

anybody have any ideas how i could more 'correctly' do this? for instance...i chose 1500 as the array size to ensure it would be big enough. could i implement some sort of counter that holds the number of primes found, and then automatically set the array size to that value?

I don't know if this approach is better or more correct than your approach but it's different.

Code:

---

#include <stdio.h>
#include <malloc.h>

int main(void)
{
    int this_number = 3, position = 0,divisor, not_prime, index;
    int *array = NULL;

    while(this_number < 10000)
    {
        divisor =  this_number >> 1;
        not_prime = 0;
        while(divisor > 1)
                {
            if(!(this_number % divisor))
            {
                divisor = !divisor;
                not_prime = 1;
            }
            else
                divisor -= 1;
        }
        if(!not_prime)
                {
            if(!position)
                array =  malloc(sizeof(int) * 1);
            else
                realloc(array, sizeof(int) * position+1);
            array[position] = this_number;
            position +=1;
        }
        this_number += 1;
    }
    for(index = 0; index < position; index++ )
    {
        if(*(array+index+1) - *(array+index) == 2)
        {
            printf ("%d and %d are prime pairs.\n",*(array+index) ,*(array+index+1) );
        }
    }
    if(array)
        free(array);
    return 0;
}

---

Code:

---

#include <malloc.h>

---

*Ahem* Mr. Rip Van Winkle, it's been 20 years, but you may want to use the correct header.

Code:

---

realloc(array, sizeof(int) * position+1);

---

This has already been panned here ad nauseum.

Checking return values is always something we like to encourage here.

And my linter doesn't like this either.

Code:

---

divisor =  this_number >> 1;

---

I stand corrected on the headers. Normally, I check all returns to keep the QA folks happy. Here on the forum, I thought I could get away with posting sloppy code. Mostly because I get a little lazy and just don't feel like writing all the additional code. I was wrong. This forum is a lot tougher than the QA types.

I'll just have to be more concientious in the future.

Thanx ;)

Bob

We tend to post with the -pedantic flag whenever possible. ;)


Quzah.