Sorting an array of structures in C

Question

Let's say I have structure like this:

typedef struct MyStruct{
  char *string1;
  int number1, number2, number3;
  char string2[11], string3[9];
  char *string4;
  char *string5;
}MyStruct;

Programs prompts user to choose by what field it should sort the data. I am having trouble thinking of a way to sort array effectively. Do I really need to write separate sorting functions for each field? There must be some other way, because writing 8 functions, where 2 would suffice, doesn't look rational.

Answer 1

Look up qsort() from <stdlib.h>. It takes a comparator function. You can write separate comparator functions for the different sort orders, but still use the standard library qsort() to do the sorting.

For example:

int ms_cmp_string1(const void *vp1, const void *vp2)
{
    const MyStruct *ms1 = vp1;
    const MyStruct *ms2 = vp2;

    int cmp = strcmp(ms1->string1, ms1->string2);
    if (cmp != 0)
        return cmp;
    else if (ms1->number1 < ms2->number1)
        return -1;
    else if (ms1->number1 > ms2->number1)
        return +1;
    //...other comparisons as required...
    else
        return 0;
}

This is a decent outline for comparators. This one sorts on string1 and then by number1. You can either write variants that sort on different fields, or devise a scheme that applies the various possible tests in an order of your choosing. But the basic outline works pretty well and is suitable for passing to qsort() without any casts necessary.