Using qsort()

[rocketman]

I'm currently trying to sort an array of type cable, which is a class.

class Cable
{
public:
	int city1,city2, cost;
};

How would i sort this array using qsort(), with length ncables, based on the cost element of the array.

[Albatross]

Wait, if you're using C++, why are you using qsort() when you can use std::sort()?

You'll probably want to write a function that takes two Cables, gets the costs from them, and return the order in which they (the elements) should go in the form of a boolean. std::sort() can take a function to replace the < operator (if it exists).
http://cplusplus.com/reference/algorithm/sort/

Good luck!

-Albatross

[rocketman]

Thanks for this. It doesn't really matter what i use, but i'm having trouble constructing the comparator, and would like some guidance on building that

[Albatross]

Hmm... does line 7 of the example on that page give you a rough idea of what to do?

-Albatross

[rocketman]

it does, but how would i specify that i want it to sort the array based on the cost element?

[Albatross]

Since your cost element is public, then can't you just use the . operator to get it from your objects and compare that?

-Albatross

[rocketman]

I'm just having trouble implementing it. For the comparator, it needs to follow int comparator ( const void * elem1, const void * elem2 );

I just don't know how to construct this to take the .cost element.

[Peter87]

If you are using qsort you need to cast the pointers to the correct type before you can access the class members.

It's much easier to use std::sort, you don't need to cast. Just define the comparator
bool comparator ( const Cable& elem1, const Cable& elem2 );

[rocketman]

Ok, so i've tried to cast the pointers and all and use quicksort.

int comparator ( const void * elem1, const void * elem2 )
{
	Cable* c1= (Cable*) &elem1;
	Cable* c2= (Cable*) &elem2;
	if(c1->cost > c2->cost) return 1;
	else return 0;
}

I've got this as the comparator, using qsort(cables,ncables, sizeof(Cable), comparator);

The problem is that it just seems to not sort all of these in any specific order at all, it just changes them around.

[Texan40]

For simplicity's sake perhaps just use references to Cable objects?

int maxCable( const Cable &c1, const Cable &c2)
{
  return (c1.cost < c2.cost) ? -1 : ((c1.cost > c2.cost) ? 1 : 0);
}

EDIT: After Peter87's explanation, reworked the return

[Peter87]

The problem is that the qsort comparator should return a negative number if elem1 is less than elem2, zero if both elements are equal and a positive number if elem1 is greater than elem2.

[rocketman]

Texan40, with qsort, the comparator has to be int comparator(const void *, const void *)

If it was just a regular comparator i'd be fine, but i can't seem to get this one to work.

I've currently got

int comparator ( const void * elem1, const void * elem2 )
{
	Cable* c1= (Cable*) &elem1;
	Cable* c2= (Cable*) &elem2;
	if(c1->cost < c2->cost) return -1;
	else if(c1->cost > c2->cost) return 1;
	else return 0;
}

and
qsort(cables,ncables, sizeof(Cable), comparator);

[Peter87]

&elem1 this gives you a pointer to a void pointer.

[rocketman]

Oh, I can't believe i didn't change that. Thanks, Peter87!