Swapping elements in an array of structs

Question

Say I have this struct:

struct MyStruct {
  int iID;
  int iMyNumber;
};

Then I define an array of MyStructs:

struct MyStruct msTest[3];

I'm doing a sorting operation on a struct similar to this one by looking at the ID. Now, as soon as I find out which records should be swapped to sort the array I have to do the actual swapping. I tried this:

if (iSmallest != iCntr) {
    stPTmp = &stXDB[iCntr];
    &stXDB[iCntr] = &stXDB[iSmallest];
    &stXDB[iSmallest] = &stPTmp;
}

stPTmp is defined as void *stPTmp; and iCntr and iSmallest contain the indices of the records to be swapped. My code doesn't work, but how do I fix it?

Answer 1

You need to swap elements, not pointers,

struct MyStruct stTmp;

if (iSmallest != iCntr) {
    stTmp = stXDB[iCntr];
    stXDB[iCntr] = stXDB[iSmallest];
    stXDB[iSmallest] = stTmp;
}

Not terribly efficient, but your structs are small, so its only a bit more expensive than swapping pointers.

Answer 2

You could just let someone else think about this, i.e. use qsort():

#include <stdlib.h>


int compare_struct(const void *a, const void *b)
{
  const struct MyStruct *sa = a, *sb = b;

  return (sa->iID < sb->iID) ? -1 : sa->iId > sb->iId;
}

qsort(msTest, sizeof msTest / sizeof *msTest, sizeof *msTest, compare_struct);

Note that this totally removed the need to write the swapping function. Under the hood, this might be a bit more costly (could use malloc(), almost certainly uses memcpy()), but it's way easier to write and much easier to maintain.