C array sorting using qsort?

Question

I have been stuck on this for a while and nothing seems to work.

I have a data structure:

DATA
{
  int size;
  int id;
}

And I have an array of DATA structures:

myArray = (DATA *) malloc(10 * sizeof(DATA));

Then I assign some test values:

myArray[0].size = 5;
myArray[1].size = 9;
myArray[2].size = 1;
myArray[3].size = 3;

So my starting array should look like:

5,9,1,3,0,0,0,0,0,0

Then, I call qsort(myArray,10,sizeof(DATA),comp)

Where comp is:

  int comp(const DATA * a, const DATA * b)
{
    return a.size - b.size;
}

And trust me, I tried many things with the compare function, NOTHING seems to work. I just never get any sorting that makes any sense.

Answer 1

So my starting array should look like 5, 9, 1, 3, 0, 0, 0, 0, 0, 0.

No, it really won't, at least it's not guaranteed to.

If you want zeros in there, either use calloc() to zero everything out, or put them in yourself. What malloc() will give you is a block of the size required that has indeterminant content. In other words, it may well have whatever rubbish was in memory beforehand.

And, on top of that, a and b are pointers in your comp function, you should be using -> rather than . and it's good form to use the correct prototype with casting.

And a final note: please don't cast the return from malloc in C - you can get into problems if you accidentally forget to include the relevant header file and your integers aren't compatible with your pointers.

The malloc function returns a void * which will quite happily convert implicitly into any other pointer.

---

Here's a complete program with those fixes:

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

typedef struct {int size; int id;} DATA;

int comp (const void *a, const void *b) {
    return ((DATA *)a)->size - ((DATA *)b)->size;
}

int main (void) {
    int i;
    DATA *myArray = malloc(10 * sizeof(DATA));
    myArray[0].size = 5;
    myArray[1].size = 9;
    myArray[2].size = 1;
    myArray[3].size = 3;
    for (i = 4; i < 10; i++)
        myArray[i].size = 0;

    qsort (myArray, 10, sizeof(DATA), comp);
    for (i = 0; i < 10; i++)
        printf ("%d ", myArray[i].size);
    putchar ('\n');

    return 0;
}

The output:

0 0 0 0 0 0 1 3 5 9