Extending inner for loop calls indefinitely [closed] - c

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The following code snippet will print every 4 character long combination (without repetition) of the elements of an array.
for (int i = 0; i < len; i++)
for (int j = i + 1; j < len; j++)
for (int k = j + 1; k < len; k++)
for (int l = k + 1; l < len; l++)
printf("%c%c%c%c\n", arr[i], arr[j], arr[k], arr[l]);
My problem is I don't know how to extend this to a general function (ie print every N character long combination). How can I make a function do the same thing but be called like this:
combinationPrint(array, numberOfForLoops); // With other params if needed

Answer 1

The recursive version of your function would work like this:
void recur (char* arr, int i, int len, char *x, int k, int n) {
if (k==n) { // the last inner loop
x[k]=0;
printf ("%s\n", x);
}
else {
for (int j=i+1; j<len; j++) { // recursive loop
x[k]=arr[j];
recur (arr, j, len, x, k+1, n); // call recursion for an inner loop
}
}
}
In this recursion, arr and len correspond to your definition and n is the depth of the loops you want to achieve (4 in your non recursive version).
The trick is to use a null terminated array of n+1 chars, that you keep across the recursion and build the string that you want to print at the last level. i is then the starting position of the loop and k is the current level of recursion.
You would call this:
recur (arr, -1, len, out, 0, 4 );
Online demo

Answer 2

Without recursion, you can use this (len = length of the array) :
int i, j, w, x;
for(i=0; i<pow(len,len); i++){ //n^n possibilities / combinaisons
w = i;
for(j=0; j<len; j++){ //Show the combinaison
x = w%len; //We have juste to calculate the correct position with some modulos
printf("%c", array[x]);
w = w/len;
}
printf("\n");
}
For exemple with this implementation :
#include <stdio.h>
#include <math.h>
int main(){
int array[] = {1,2,3};
int len = 3;
int i, j, w, x;
for(i=0; i<pow(len,len); i++){
w = i;
for(j=0; j<len; j++){
x = w%len;
printf("%d", array[x]);
w = w/len;
}
printf("\n%d\n", i);
}
}
You should have :
111
211
311
121
221 [...]
133
233
333

Answer 3

Well, instead of N loops - you just need one. Within this main loop you have to have just two inner loops for incrementing array of indices and for printing values:
int increment(int* index, int N, int len)
{
for (int i = N - 1; i >= 0; --i)
{
++index[i];
if (index[i] < len)
return 1;
index[i] = 0;
}
return 0;
}
So - the main loop:
int index[N] = {};
do
{
// print
for (int i = 0; i < N; ++i)
printf("%c", arr[index[i]]);
printf("\n");
} while (increment(index, N, len));