qsort problem (ish)

Well I was reading up on function pointers - all very interesting - and the example it showed me was the use of the qsort function. So, I closed the page, opened my IDE and fiddled. The result:

Code:

---


#include <algorithm>
#include <iostream>
int Comparison (const void* _a, const void* _b)
{
        const int a = (const int) _a;
        const int b = (const int) _b;
        if (a > b)
                return 1;
        else {
                if (a == b)
                        return 0;
                else
                        return -1; // a < b
        }
}
void walk_array (int a[], int length)
{
        for (int i=0; i<length; i++)
                std::cout << a[i] << std::endl;
}
 
int main()
{
        int data[10] = { 3, 7, 5, 9, 11, 90, 44, 12, 54, 47 };
        qsort ((void*) data, 10, sizeof (int), &Comparison);
        walk_array (data, 10);
        return 0;
}

---

It compiles and runs fine, I just get this output, apparently sorted, but not in any particular order. Weird:

Code:

---


90
3
5
9
11
7
44
12
54
47

---

Very unsorted.

Ah I just noticed something. Is my compare function returning the wrong number(s)?

The numbers are fine, your casting is a bit off. Plus you need to leave out the & operator when passing the function as an argument.

Code:

---

#include <algorithm>
#include <iostream>

int Comparison (const void* _a, const void* _b)
{
        const int a = *((const int*)_a);
        const int b = *((const int*)_b);

        if (a > b) return 1;
       
        if (a == b) return 0;
               
        return -1; // a < b

}
void walk_array (int a[], int length)
{
        for (int i=0; i<length; i++)
        {
                std::cout << a[i] << std::endl;
        }
}
 
int main()
{
        int data[10] = { 3, 7, 5, 9, 11, 90, 44, 12, 54, 47 };

        qsort( data, 10, sizeof(data[0]), Comparison );

        walk_array( data, 10 );

        return 0;
}

---

Thanks that did the job!

Oh and thanks for clearing up my Comparison function; it was a little messy.