Thread: Urgent Help - Qsort

Originally Posted by it01y2

I have trying to work this out for days, but cant get my head around it.

Show us something you've tried and someone will tell you why it doesn't work. Or ask a more specific question about the compare function you use with qsort (that is, the 4th argument ). I'm not going to write the code for you.

This is my qsort function:

Code:

for(j=0; j < 22; j++)
{
    qsort(p[j].person,p[j].number,sizeof(struct queue),struct_cmp);
}

And my (struct_cmp) comparison function consists of:

Code:

int struct_cmp(const void *a, const void *b) /* Compare integer function */
{	
    cc *p1 = (cc*)a;
    cc *p2 = (cc*)b;

	if(p1->person->age > p2->person->age)
		return 1;
	else
		return 0;
}

Im guessing this is total rubbish, i just get stuck with pointers and structs.

Okay, qsort arguments are (your second one is definitely wrong):

1) a pointer to your struct array
2) the number of elements in the array (how bout 400?)
3) the sizeof one element
4) the compare function pointer

If you are using a struct array, you cannot use a compare function which accepts two ints. It must accept two structs and compare the relevent fields within them.

Originally Posted by it01y2

Code:

qsort(q[j].person,400,sizeof(struct queue),struct_cmp);

I am not sure how to get the sizeof one element?

sizeof(struct pers)? I am not awake yet...

Originally Posted by it01y2

Code:

qsort(q[j].person,400,sizeof(struct queue),struct_cmp);

I am not sure how to get the sizeof one element?

Since q[j].person is an array of struct pers, then each element is sizeof(struct pers), not sizeof(struct queue). qsort needs to know how big each unit of the array is, since the pointer (1st arg) is a void type, so the "kind" of array cannot be determined by qsort internally. It has to send 2X "3rd arg" bytes of data at a time to the compare function.

Originally Posted by it01y2

In the function im going to have to pass the struct by pointer into the compare function, then access the childs to do a comparison im guessing so?

Add a cast to the correct type and I think that you are there...

God I hope my caffeine kicks in soon...

Originally Posted by it01y2

I have tried to create the compare function but im doing something wrong:

I would stick some printf's in there, or use a debugger, to check the values and see what's happening.