Program crashes when `if (variable % 2 == 0)`

Question

I'm writing a program that finds perfect numbers. Having read about these perfect numbers I came across a list of them: List of perfect numbers. At the moment the output is:

28         // perfect
496        // perfect
8128       // perfect
130816     // not perfect
2096128    // not perfect
33550336   // perfect

I decided to create array and put it with numbers, which divide the number wholly (without the rest). So I will be able to verify if it is a perfect number or not by adding all elements of the array. But app crashes and I cannot understand why:

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

int main()
{
    unsigned long number;
    unsigned long arr2[100] = {0};
    int k = 0;

    for ( number = 0; number <= 130816; number++ )
        if ( 130816 % number == 0 )
            arr2[k++] = number;

    for ( k = 0; k < 100; k++ )
        printf("%lu", arr2[k]);

    return 0;
}

Answer 1

You are doing modulus zero here:

if ( 130816 % number == 0 )

which is undefined behavior. If you start your for loop at 1 instead it should fix that issue. However, since N % 1 == 0 for all N, you probably need to start at 2.

From the C99 standard, 6.5.5 /5 (unchanged in C11):

The result of the / operator is the quotient from the division of the first operand by the second; the result of the % operator is the remainder. In both operations, if the value of the second operand is zero, the behavior is undefined.

Answer 2

You are dividing by zero when number=0;

138816 % number involves division and a remainder.