How to use malloc properly in C?

Question

I'm writing a program in which I want to find all abundant numbers until 28124. I can do it with a static array but I want to improve my code and adjust it with a dynamic way. Here is my code:

    
    #define N 28124
 
    bool is_abundant(int number)
    {
        int i;
        int sum=0;
        for(i=1;i<=number/2;i++)
        {
            if(number % i == 0)
                sum += i;
            if(sum > number)    return true;
        }
    
        return false;
    }

    int main(int argc, char **argv)
    {
        clock_t start = clock();
    
        int i, j=0;
        int *abundants = NULL;  
    
        for(i=12;i<N;i++)
        {
            if(is_abundant(i))
            {
                abundants = (int*)malloc(sizeof(int));
                assert(abundants);
                abundants[j] = i;
                j++;
            }
        }

        for(i=0;i<j;i++)
        {
            printf("%d ", abundants[i]);
        }

        clock_t end = clock();   
        double time = (double) (end - start)/CLOCKS_PER_SEC;
        printf("Execution time: %lf\n", time);
        return 0;
    }

The program aborts due to assert failure but how can I change it so as to works properly in a dynamic way?

Answer 1

You need to multiply sizeof(int) by the number of ints you want to allocate. Otherwise you just allocate a single element.

You also need to use realloc() so you grow the existing array rather than allocating a new array.

    for(i=12;i<N;i++)
    {
        if(is_abundant(i))
        {
            abundants = realloc(abundants, (j+1) * sizeof(int));
            assert(abundants);
            abundants[j] = i;
            j++;
        }
    }