Question

My code is to sort a structure by just one field.

It is strange that it works when the length is 8 but not in
9.Why is that and what is wrong?

struct node
{
   int key;//I need to sort by the key
   int val;
};
int comp(const void *a, const void *b)
{
return ((struct node *)a)->key > ((struct node *)b)->key;
}
int main()
{
    int i;
    struct node *a;
    a = malloc(10 * sizeof *a);
    /*I have 8 elements*/
    for (i = 0; i < 6; i++)
         a[i].key = 22;
    a[6].key = 21;
    a[7].key = 20;
    a[8].key = 10;
    /*Before sorting, I print it first*/
    for (i = 0; i < 9; i++)
    printf("%3d", a[i].key);
    printf("\n");
    qsort(a, 9, sizeof(struct node), comp);
    /*The sorted answer*/
    for (i = 0; i < 9; i++)
       printf("%3d", a[i].key);
    printf("\n");
    free(a);
    return 0;
}

Output is:

22 22 22 22 22 22 21 20 10

10 22 22 22 22 22 21 20 22

But when I change the length to 8, it works.

Answer 1

What do you mean by “length”?

• You allocate space for 10 elements.
• You sort 9 of them.
• You print 9 of them.

This code is very confusing. You need to have a single well-defined idea of how many elements you’re using, and use the same everywhere. Hint: it should not be a literal number.

Also, your comparison function is wrong, it needs to return -1, 0 or 1 for elements being less than, equal to, or greater than respectively. Your function wil only return 0 or 1.