Generating subsets of a set in C(Non recursive) - c

Question

I am trying to generate all the subsets of a set in a non recursive manner. This is the code:
#include <stdio.h>
main() {
int no_of_element, no_of_subset, i, j, start, index, a[50], x;
printf("Enter the size of main set :");
scanf("%d", &no_of_element);
printf("Enter the elements of main set :");
for (x = 0; x < no_of_element; x++)
scanf("%d", &a[x]);
printf("The subsets are :\n");
for (no_of_subset = 1; no_of_subset <= no_of_element; no_of_subset++) {
for (start = 0; start <= no_of_element - no_of_subset; start++) {
if (no_of_subset == 1)
printf("%d\n", a[start]);
else {
index = start + no_of_subset - 1;
for (j = index; j < no_of_element; j++) {
for (i = start; i < index; i++) {
printf("%d\t", a[i]);
}
printf("%d\n", a[j]);
}
}
}
}
}
This is the output:
Enter the size of main set :4
Enter the elements of main set :1
2
3
4
The subsets are :
1
2
3
4
1 2
1 3
1 4
2 3
2 4
3 4
1 2 3
1 2 4
2 3 4
1 2 3 4
This code isn't generating the subset (1,3,4). Could someone please help me fix it ?

Answer 1

Just for fun, I dit it from scratch.
The trick is to let a counter go through all the possible subsets, each being a combination of its bits.
In case, use unsigned long int to handle sets of up to 64 elements:
#include <stdio.h>
void print_subset(int subset, int *set) {
int pos=0;
printf("Subset %d = ", subset);
while (subset) {
if (subset & 1) {
printf("%d ", set[pos]);
}
subset >>= 1;
pos++;
}
printf("\n");
}
int main()
{
int no_of_element,no_of_subset,x,a[50];
printf("Enter the size of main set :");
scanf("%d",&no_of_element);
printf("Enter the elements of main set :");
for(x=0;x<no_of_element;x++)
scanf("%d",&a[x]);
no_of_subset= (1 << no_of_element);
printf("There are %d subsets\n", no_of_subset);
for (; no_of_subset--;) {
print_subset(no_of_subset, a);
}
}
When run it produces
Enter the size of main set :4
Enter the elements of main set :1
2
3
4
There are 16 subsets
Subset 15 = 1 2 3 4
Subset 14 = 2 3 4
Subset 13 = 1 3 4
Subset 12 = 3 4
Subset 11 = 1 2 4
Subset 10 = 2 4
Subset 9 = 1 4
Subset 8 = 4
Subset 7 = 1 2 3
Subset 6 = 2 3
Subset 5 = 1 3
Subset 4 = 3
Subset 3 = 1 2
Subset 2 = 2
Subset 1 = 1
Subset 0 =

Answer 2

#include <stdio.h>
#include <stdlib.h>
void print(size_t n, int a[n]){
size_t i;
for(i=0; i < n; ++i){
if(i)
putchar(',');
printf("%d", a[i]);
}
putchar('\n');
}
void subset(size_t n, int a[n], size_t size){
int *out = malloc(size * sizeof(*out));
int *level = malloc((size+1)*sizeof(int));
int *index = malloc((size+1)*sizeof(int));
int sp = 0;
level[sp] = 0;
index[sp] = 0;
redo:
while(index[sp] < n){
out[level[sp]] = a[index[sp]];
level[sp+1] = level[sp] + 1;
index[sp+1] = index[sp] + 1;
if(level[++sp] == size){
print(size, out);
--sp;
} else
goto redo;//this means recursive call
++index[sp];
}
if(sp>0){
++index[--sp];
goto redo;
}
free(level);
free(index);
free(out);
}
int main(void){
int a[] = {1, 2, 3, 4};
size_t i, n = sizeof(a)/sizeof(*a);
for(i = 1; i <= n; ++i)
subset(n, a, i);
return 0;
}
result:
1
2
3
4
1,2
1,3
1,4
2,3
2,4
3,4
1,2,3
1,2,4
1,3,4
2,3,4
1,2,3,4