How to write a compare function for qsort from stdlib?

Question

I have a structure:

struct pkt_
{
  double x;
  double y;
  double alfa;
  double r_kw;
};

typedef struct pkt_ pkt;

A table of these struc

Answer 1

There are two parts to the problem - how to write the code, and how to compare the packet types. You must ensure you always return a value. Your code should also always be such that:

porownaj(&pkt_a, &pkt_b) == -porownaj(&pkt_b, &pkt_a)

Your outline comparison does not handle cases such as:

pkt_a->alfa >  pkt_b->alfa && pkt_a->r_kw <= pkt_b->r_kw
pkt_a->alfa <  pkt_b->alfa && pkt_a->r_kw >= pkt_b->r_kw
pkt_a->alfa == pkt_b->alfa && pkt_a->r_kw != pkt_b->r_kw

There is one more problem - is it appropriate to compare floating point values for exact equality? That will depend on your application.

Mechanically, you have to convert the const void pointers to const structure pointers. I use the explicit cast - C++ requires it, and I try to make my code acceptable to a C++ compiler even when it is really C code.

int porownaj(const void *vp1, const void *vp2)
{
     const pkt *pkt_a = (const pkt *)vp1;
     const pkt *pkt_b = (const pkt *)vp2;

     if (pkt_a->alfa >  pkt_b->alfa && pkt_a->r_kw >  pkt_b->r_kw) return 1;
     if (pkt_a->alfa == pkt_b->alfa && pkt_a->r_kw == pkt_b->r_kw) return 0;
     if (pkt_a->alfa <  pkt_b->alfa && pkt_a->r_kw <  pkt_b->r_kw) return -1;
     return 0;
 }

This does not deal with the bits that I cannot resolve since I am not party to the necessary information. Note that, in general, multi-dimensional objects (such as complex numbers, or (x,y) or (x,y,z) coordinates) cannot simply be compared for greater than or less than or equal to.