I've looked around online for an non-recursive k-combinations algorithm, but have had trouble understanding all of the reindexing involved; The code I've found online is not commented well, or crashes.
For example, if I have the collection, {'a', 'b', 'c', 'd', 'e'} and I want to find a 3 combinations; ie,
abc
abd
abe
acd
ace
ade
bcd
bce
bde
cde
How can I implement an algorithm to do this? When I write down the general procedure, this it is clear. That is; I increment the last element in a pointer until it points to 'e', increment the second to last element and set the last element to the second to last element + 1, then increment the last element again until it reaches 'e' again, and so on and so forth, as illustrated by how I printed the combinations. I looked at Algorithm to return all combinations of k elements from n for inspiration, but my code only prints 'abc'. Here is a copy of it:
#include <stdio.h>
#include <stdlib.h>
static void
comb(char *buf, int n, int m)
{
// Initialize a pointer representing the combinations
char *ptr = malloc(sizeof(char) * m);
int i, j, k;
for (i = 0; i < m; i++) ptr[i] = buf[i];
while (1) {
printf("%s\n", ptr);
j = m - 1;
i = 1;
// flag used to denote that the end substring is at it's max and
// the j-th indice must be incremented and all indices above it must
// be reset.
int iter_down = 0;
while((j >= 0) && !iter_down) {
//
if (ptr[j] < (n - i) ) {
iter_down = 1;
ptr[j]++;
for (k = j + 1; k < m; k++) {
ptr[k] = ptr[j] + (k - j);
}
}
else {
j--;
i++;
}
}
if (!iter_down) break;
}
}
int
main(void)
{
char *buf = "abcde";
comb(buf, 5, 3);
return 1;
}