This is some code I created in association with a question SO 4203-5818 (that actually has little to do with sorting). It is for sorting arrays of 3 elements (sides of triangles in the other question), but isn't an answer to that question.
Assuming that you can/want to sort the data rather than just list it in sorted order, you could use one of the sort_3() variants shown in the code below, suitably adapted to your circumstances.
It includes test code which tests every combination of 3 values selected from { 0, 1, 2 }. It uses 3 comparisons and up to 3 swaps to sort the data.
/* Sort 3 items - fixed comparisons */
/* Based on code for SO 4203-5818 */
#include
#include
static inline void swap(int *a, int *b) { int t = *a; *a = *b; *b = t; }
/* Interesting: 4 of 6 permutations work, 2 fail */
#if P_01_02_12 + P_01_12_02 + \
P_02_01_12 + P_02_12_01 + \
P_12_01_02 + P_12_02_01 > 1
#error Too many of the control macros (P_[01][12]_[01][12]_[01][12]) are set
#endif
#if P_01_02_12 + P_01_12_02 + \
P_02_01_12 + P_02_12_01 + \
P_12_01_02 + P_12_02_01 == 0
#define P_02_01_12 1
#endif
#define P_STRING(x) static char const variant[] = #x
#ifdef P_01_02_12
P_STRING(P_01_02_12);
#endif
#ifdef P_01_12_02
P_STRING(P_01_12_02);
#endif
#ifdef P_02_01_12
P_STRING(P_02_01_12);
#endif
#ifdef P_02_12_01
P_STRING(P_02_12_01);
#endif
#ifdef P_12_01_02
P_STRING(P_12_01_02);
#endif
#ifdef P_12_02_01
P_STRING(P_12_02_01);
#endif
#if P_02_01_12
/* Working */
static void sort_3(int *a)
{
if (a[0] > a[2]) swap(&a[0], &a[2]);
if (a[0] > a[1]) swap(&a[0], &a[1]);
if (a[1] > a[2]) swap(&a[1], &a[2]);
}
#endif /* 0 */
#if P_12_01_02
/* Triggers assertion */
static void sort_3(int *a)
{
if (a[1] > a[2]) swap(&a[1], &a[2]);
if (a[0] > a[1]) swap(&a[0], &a[1]);
if (a[0] > a[2]) swap(&a[0], &a[2]);
}
#endif /* 0 */
#if P_01_12_02
/* Triggers assertion */
static void sort_3(int *a)
{
if (a[0] > a[1]) swap(&a[0], &a[1]);
if (a[1] > a[2]) swap(&a[1], &a[2]);
if (a[0] > a[2]) swap(&a[0], &a[2]);
}
#endif /* 0 */
#if P_12_02_01
/* Working */
static void sort_3(int *a)
{
if (a[1] > a[2]) swap(&a[1], &a[2]);
if (a[0] > a[2]) swap(&a[0], &a[2]);
if (a[0] > a[1]) swap(&a[0], &a[1]);
}
#endif /* 0 */
#if P_02_12_01
/* Working */
static void sort_3(int *a)
{
if (a[0] > a[2]) swap(&a[0], &a[2]);
if (a[1] > a[2]) swap(&a[1], &a[2]);
if (a[0] > a[1]) swap(&a[0], &a[1]);
}
#endif /* 0 */
#if P_01_02_12
/* Working */
static void sort_3(int *a)
{
if (a[0] > a[1]) swap(&a[0], &a[1]);
if (a[0] > a[2]) swap(&a[0], &a[2]);
if (a[1] > a[2]) swap(&a[1], &a[2]);
}
#endif /* 0 */
static void print_3(int *a)
{
const char *pad = "";
for (int i = 0; i < 3; i++)
{
printf("%s%d", pad, a[i]);
pad = " ";
}
}
static void check_3(int *a)
{
assert(a[0] <= a[1]);
assert(a[1] <= a[2]);
}
int main(void)
{
printf("Variant: %s\n", variant);
fflush(stdout);
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
for (int k = 0; k < 3; k++)
{
int a[3] = { i, j, k };
print_3(a);
fputs(" : ", stdout);
sort_3(a);
print_3(a);
putchar('\n');
check_3(a);
}
}
}
return 0;
}
You can find this code in my SOQ (Stack Overflow Questions) repository on GitHub as files st13.c (and st17.c and st19.c for variants on the idea) in the src/so-4203-5818 sub-directory. Note that st19 (and the test script test.st19.sh) show that there are 6 possible working sorts — 01:02:12, 01:12:01, 02:01:12, 02:12:01, 12:01:12, 12:02:01 — out of a 27 (33) possible combinations. (The 01:12:01 and 12:01:12 working variants are not demonstrated in st13 but can be demonstrated by st19.)
If you must sort 3 variables with disjoint names, then you can adapt any of the working array-sorting functions to work inline. For example, using the P_01_02_12 variant to sort v0, v1, v2, you'd end up with:
if (v0 > v1) swap(&v0, &v1);
if (v0 > v2) swap(&v0, &v2);
if (v1 > v2) swap(&v1, &v2);
