Power set generated by bits

Question

I have this code which generates power set of an array of size 4 (number is just example, less combinations to write...).

#define ARRAY_SIZE 4


unsigned int i,          


        

Answer 1

Use a buffer array and then sort it using qsort ?

I used a i_max*5 elements array : 4 unsigned int for the values (0 means empty, 1 to 4 otherwise) and a final unsigned int for the subset length. Then, you just have to roll with your custom comparator. If you want to a more streamlined algorithm, take a look at insertion sort : http://en.wikipedia.org/wiki/Insertion_sort

Output:

1,
2,
1,2,
3,
1,3,
2,3,
1,2,3,
4,
1,4,
2,4,
1,2,4,
3,4,
1,3,4,
2,3,4,
1,2,3,4,

Sorting
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,

Code:

#include 
#include 
#include 

int comp (const void * elem1, const void * elem2) {
    unsigned int *f = (unsigned int*)elem1;
    unsigned int *s = (unsigned int*)elem2;
    unsigned int i;


    //printf("%d,%d,%d,%d,%d\n", f[0],f[1],f[2],f[3],f[4]);
    //printf("%d,%d,%d,%d,%d\n", s[0],s[1],s[2],s[3],s[4]);
    printf("\n");

    // Size comparison     
    if( f[4] > s[4] ) return 1;
    if( f[4] < s[4] ) return -1;

    // Value comparison
    for ( i = 0; i < 4; i++)
    {
        if (f[i] > s[i]) return  1;
        if (f[i] < s[i]) return -1;
    }

    return 0;
}

int main(int argc, char *argv[])
{
    unsigned int i, j, bits, i_max = (1U << 4);
    unsigned int *array;
    unsigned int index;

    array = calloc( 5*i_max, sizeof(unsigned int));

    // Non-sorted array
    for (i = 0; i < i_max ; ++i) {

        // Zero init for array
        memset(array + i*5, 0, 5*sizeof(*array));
        index = 0;

        for (bits = i, j = 0; bits; bits >>= 1, ++j) {

        if (bits & 1)
            {
                // Storage
                array[i*5 + index]   = j+1;

                index = (index + 1) % 4; // avoid buffer overflows
                array[i*5 + 4]   = array[i*5 + 4] + 1 ; // increment subset length

                //printf("%d,", array[i*5 + index - 1]);
            }
        }
        printf("\n");
    }

    // Print original array, without the zeros
    for (i =0; i < i_max; i++)
    {
        for(j = 0; j < 4 ; j++)
        {
            if( array[i*5 + j] )
                printf("%d,", array[i*5 + j]);
        }
        printf("\n");
    }


    printf ("Sorting\n");

    qsort (array, i_max, 5*sizeof(unsigned int), comp);


    // Print the sorted array, without the zeros
    for (i =0; i < i_max; i++)
    {
        for(j = 0; j < 4 ; j++)
        {
            if( array[i*5 + j] )
                printf("%d,", array[i*5 + j]);
        }
        printf("\n");
    }
    free(array);

    return 0;
}