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

John has already answered your question, but to sort your structs, you can use the standard library qsort() function:

#include <stdlib.h>
#include <stdio.h>

struct MyStruct {
    int iID;
    int iMyNumber;
};

/* comparison function, should return < 0, > 0 or == 0
   if a < b, a > b or a == b respectively.  Used by qsort */
static int comp_mystruct(const void *a, const void *b);

/* utility function to print an array of our struct */
static void print_mystruct(const void *start, size_t n);

int main(void)
{
    /* some data */
    struct MyStruct data[] = {
        { 1, 10 },
        { 5, 50 },
        { 2, 20 },
        { -3, 100 }
    };
    size_t ndata = sizeof data / sizeof data[0];

    /* before sorting */
    print_mystruct(data, ndata);
    putchar('\n');

    /* sort the array now */
    qsort(data, ndata, sizeof data[0], comp_mystruct);

    /* after sorting */
    print_mystruct(data, ndata);

    return 0;
}

static void print_mystruct(const void *start, size_t n)
{
    size_t i;
    const struct MyStruct *s = start;
    for (i=0; i < n; ++i) {
        printf("% 3d % 3d\n", s[i].iID, s[i].iMyNumber);
    }
}

static int comp_mystruct(const void *a, const void *b)
{
    const struct MyStruct *sa = a;
    const struct MyStruct *sb = b;
    if (sa->iID > sb->iID) {
        return 1;
    } else if (sa->iID < sb->iID) {
        return -1;
    } else {
        return 0;
    }
}

The output of the program is:

  1  10
  5  50
  2  20
 -3  100

 -3  100
  1  10
  2  20
  5  50

The advantage is that qsort() is standard, and you can use it to sort anything.

Answer 3

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.