floating point exception trying to find prime numbers

Question

I have a program that generates prime numbers. This code works fine when i want the first 100 and 200 primes, but puts out a Floating point exception whenever I use a value above 300 for total. The problem seems to be in the array, but I don't understand what is happening.

#include<stdio.h>

int main()
{
    int total = 500;
    int primes[total];
    primes[0] = 2;
    int max = 1;
    int current = 3;
    int index = 0;
    printf("%d\n",2);
    while(max != total)
    {
        for(index = 0; index <= max + 1; index++)
        {
            if(index == max + 1){
                primes[index] = current;
                printf("%d\n",current);
                max = max + 1;
                current = current + 1;
                break;
            } else {
                if(current % primes[index] == 0){
                    current = current + 1;
                    break;
                }
            }
        }
    }
}

Small performance tips: `current = current + 2;` is fine, since all even numbers are not prime for sure; And you can replace `if (index == max + 1)` with something like `if (index == max + 1 || primes[index] * primes[index] > current)`. — FreeNickname, Sep 04 '14 at 22:28
First time in the `for()` loop, `index` goes from 0 to 2. When `index == 1`, `if(current % primes[1] == 0)` is executed. `primes[1]` never initialized. — chux - Reinstate Monica, Sep 04 '14 at 23:56

score 4 · Accepted Answer · answered Sep 04 '14 at 22:15

4

You have to ensure the expression current % primes[index] is never evaluated with a 0 value for primes[index]. A 0 value as the right operand of % operator invokes undefined behavior in C.

answered Sep 04 '14 at 22:15

ouah

142,963
15
272
331

When I run his code adjusting the array to be +1 bigger than total for the off by 1 error on the array, and total only being 500 this never happens, may he edited his code to account for that error – Steve Sep 04 '14 at 22:58
@Steve when `index == 1` the expression `current % primes[index]` is evaluated but `primes[1]` is not initialized so if the value happens to be a `0` he gets a division by `0`. On you system you may have had a different uninitialized value for `primes[1]`. A simple way to fix this issue is to have something like `primes[1] = 3;` at the top of the program. – ouah Sep 05 '14 at 07:36

score 0 · Answer 2 · answered Sep 04 '14 at 22:22

0

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your accessing beyond the end of primes[], in the debugger you write primes[index] where index == 500.. primes[500] doesn't exist

answered Sep 04 '14 at 22:22

Steve

502
3
12

I know you want to gloss over my answer, but think about this, your corrupting your memory, reading from potentially corrupted memory and doing calculations on it, which is causing your FP exception. try: `int total = 501;` `int primes[500];` and see – Steve Sep 04 '14 at 22:48

floating point exception trying to find prime numbers

2 Answers2