Question

I would like to create a dynamic array which store permutation sequence, such that

order[0][]={1,2,3}
order[1][]={2,1,3}
order[2][]={2,3,1}

let say order[m][n], m = number of permutation, n = number of term, m and n are identified in real-time.

I did the below, and found that the pointer address is overlapping, resulting in incorrect value storage. How can do it correctly using dynamic array via double pointer?

void permute(int num_permute, int num_term, int** order) {
    int x, y;
    int term[5];

    /* debug only */
    for(y=num_term, x=0; y>0; y--, x++){
        term[x] = y;
    }
    fprintf(stderr, "\n");

    printf("order%12c", ' ');
    for (x=0; x<num_permute; ++x) {
        printf("  %-11d", x);
    }
    printf("\n");
    for(y=0; y<num_permute; y++){
        printf("%-5d%12p", y, (order+y));
        memcpy(&(order[y]), term, sizeof(term));

        for (x=0; x<num_term; x++)
            printf(" %12p", order+y+x);

        printf("\n");

    }
}

int main(){
    int y, z;
    int** x;

    x = (int*) malloc(5*5*sizeof(int*));

    permute(5, 5, x);
    printf("\n");

    printf("x   ");
    for(z=0; z<5; z++){
        printf(" %2d ", z);
    }
    printf("\n");
    for(y=0; y<5; y++){
        printf("%-4d", y);
        for(z=0; z<5; z++){
            printf(" %2d ", *(x+y+z));
        }
        printf("\n");
    }

    free(x);

    return 0;
}

Result: order[0][1] and order[1][0] point to same address… and so do others. With rows as the major axis and columns the minor:

order             0            1            2            3            4           
0     0x100100080 0x100100080  0x100100084  0x100100088  0x10010008c  0x100100090
1     0x100100084 0x100100084  0x100100088  0x10010008c  0x100100090  0x100100094
2     0x100100088 0x100100088  0x10010008c  0x100100090  0x100100094  0x100100098
3     0x10010008c 0x10010008c  0x100100090  0x100100094  0x100100098  0x10010009c
4     0x100100090 0x100100090  0x100100094  0x100100098  0x10010009c  0x1001000a0

x     0   1   2   3   4 
0     5   5   5   5   5 
1     5   5   5   5   4 
2     5   5   5   4   3 
3     5   5   4   3   2 
4     5   4   3   2   1 

Answer 1

Source Code:
The code will be something like:

#include <stdlib.h>

int **array;
array = malloc(nrows * sizeof(int *));
if(array == NULL)
{
     fprintf(stderr, "out of memory\n");
     /*exit or return*/
}
for(i = 0; i < nrows; i++)
{
    array[i] = malloc(ncolumns * sizeof(int));
    if(array[i] == NULL)
    {
          fprintf(stderr, "out of memory\n");
         /*exit or return*/
    }
}

Concept:

array is a pointer-to-pointer-to-int: at the first level, it points to a block of pointers, one for each row. That first-level pointer is the first one to be allocated; it has nrows elements, with each element big enough to hold a pointer-to-int, or int *. If the allocation is successful then fill in the pointers (all nrows of them) with a pointer (also obtained from malloc) to ncolumns number of ints, the storage for that row of the array.

Pictorial Depiction:

It is simple to grasp if you visualize the situation as:

Taking this into account, the sample code could be rewritten as:

void permute(int num_permute, int num_term, int** order) {
    int x, y;
    int term[5];
    int* ptr = NULL;

    for (y=num_term, x=0; y>0; y--, x++) {
        term[x] = y;
    }
    printf("\n");

    printf("order%12c", ' ');
    for (x=0; x<num_permute; ++x) {
        printf(" %2d ", x);
    }
    printf("\n");
    for (y=0; y<num_permute; y++) {
        ptr = order[y];
        memcpy(ptr, term, sizeof(term));

        printf("%-5d%12p", y, ptr);
        for (x=0; x<num_term; x++) {
            printf(" %2d ", ptr[x]);
        }
        printf("\n");
    }
}

int main() {
    int y, z;
    int** x = NULL;
    int num_term = 5;
    int num_permutation = 5;
    int* pchk = NULL;

    x = (int**) malloc(num_permutation * sizeof(int*));

    for (y=0; y<num_permutation; y++){
        x[y] = (int*) malloc(num_term * sizeof(int));
        printf("x[%d]: %p\n", y, x[y]);
    }

    permute(num_permutation, num_term, x);

    printf("\nx:  ");
    for(z=0; z<5; z++){
        printf(" %2d ", z);
    }
    printf("\n");

    for(y=0; y<num_permutation; y++){
        pchk = x[y];
        printf("%-4d", y);
        for(z=0; z<num_term; z++){
            printf(" %2d ", pchk[z]);
        }
        printf("\n");
    }

    for (y=0; y<num_permutation; y++) {
        free(x[y]);
    }
    free(x);

    return 0;
}