Sorting an array of structures in C - 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.

Answer 2

You don't need to write 8 functions if only 2 are needed. Build your own qsort function and send a last parameter containing the member offset to the compare function, then, in your compare function, cast pointer + offset to the right type.
Something like:
int comp_int(const void *pa, const void *pb, size_t offset)
{
const int *a = (const int *)((const char *)pa + offset);
const int *b = (const int *)((const char *)pb + offset);
return *a - *b;
}
int comp_string(const void *pa, const void *pb, size_t offset)
{
const char *a = (const char *)pa + offset;
const char *b = (const char *)pb + offset;
return strcmp(a, b);
}
void swap(void *v[], int a, int b)
{
void *temp;
temp = v[a];
v[a] = v[b];
v[b] = temp;
}
void sort(void *v[], int left, int right, size_t offset, int (*comp)(const void *, const void *, size_t))
{
int i, last;
if (left >= right) return;
swap(v, left, (left + right) / 2);
last = left;
for (i = left + 1; i <= right; i++) {
if ((*comp)(v[i], v[left], offset) < 0)
swap(v, ++last, i);
}
swap(v, left, last);
sort(v, left, last - 1, offset, comp);
sort(v, last + 1, right, offset, comp);
}
offsetof can help

Answer 3

Here is a sample of using qsort from my another answer:
struct stringcase { char* string; void (*func)(void); };
void funcB1();
void funcAzA();
struct stringcase cases [] =
{ { "B1", funcB1 }
, { "AzA", funcAzA }
};
struct stringcase work_cases* = NULL;
int work_cases_cnt = 0;
// comparator function
int stringcase_cmp( const void *p1, const void *p2 )
{
return strcasecmp( ((struct stringcase*)p1)->string, ((struct stringcase*)p2)->string);
}
// prepare the data for searching
void prepare() {
// allocate the work_cases and copy cases values from it to work_cases
qsort( cases, i, sizeof( struct stringcase ), stringcase_cmp );
}

Answer 4

If you're using the GNU C library, there's an extension called qsort_r() that lets you pass an extra parameter to the comparison function.

Answer 5

Using some macros:
#include <stdio.h>
#include <stdlib.h>
struct data {
int x, y, z;
};
#define comp(member) comp_##member
#define comp_build(member) \
int comp_##member(const void *pa, const void *pb) \
{ \
const struct data *a = pa, *b = pb; \
return (a->member < b->member) ? -1 : (a->member > b->member); \
}
comp_build(x)
comp_build(y)
comp_build(z)
int main(void)
{
#define ROWS 3
struct data v[] = {
{3, 2, 1},
{1, 3, 2},
{2, 1, 3}
};
int i;
puts("Unsorted");
for (i = 0; i < ROWS; i++) printf("%d %d %d\n", v[i].x, v[i].y, v[i].z);
qsort(v, ROWS, sizeof(struct data), comp(x));
puts("Sorted by x");
for (i = 0; i < ROWS; i++) printf("%d %d %d\n", v[i].x, v[i].y, v[i].z);
puts("Sorted by y");
qsort(v, ROWS, sizeof(struct data), comp(y));
for (i = 0; i < ROWS; i++) printf("%d %d %d\n", v[i].x, v[i].y, v[i].z);
puts("Sorted by z");
qsort(v, ROWS, sizeof(struct data), comp(z));
for (i = 0; i < ROWS; i++) printf("%d %d %d\n", v[i].x, v[i].y, v[i].z);
return 0;
}