prime pairs from 3 - 10,000

This is a discussion on prime pairs from 3 - 10,000 within the C Programming forums, part of the General Programming Boards category; i found a set of beginner's problems to solve...i came up with a solution to this one: print out all ...

  1. #1
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    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?

  2. #2
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    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;
    }

  3. #3
    Just Lurking Dave_Sinkula's Avatar
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    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;
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  4. #4
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    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

  5. #5
    ATH0 quzah's Avatar
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    We tend to post with the -pedantic flag whenever possible.


    Quzah.
    Hope is the first step on the road to disappointment.

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