Thread: 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.