How to write a comparator function for qsort for a 2D array?

Question

I have an n*2 sized array. I want to sort them using qsort based on their value of the 2nd column.

#include<stdio.h>
int cmp(const int **a, const int **b) {
    //return 0;
    //return *a[1] - *b[1]
    // return *a - *b;
}
int main()
{
    int t; // test cases
    scanf("%d", &t);
    for(int i=0; i<t; i++)  {
        int n;
        scanf("%d", &n); // size of array
        int **arr = (int **)malloc(n * sizeof(int *)); 

        for(int j =0; j< n; j++) {
            arr[j] = (int *) malloc(2*sizeof(int));
        }
        for(int j =0; j< 2; j++) {
            for(int k =0; k< n; k++) {
              scanf("%d", &arr[k][j]);
            }
        }
        for(int k =0; k< n; k++) {
            for(int j =0; j<= 1; j++) {
             printf("%d\t", arr[k][j]);
            }
            printf("\n");
        }
       // qsort(arr, n, sizeof(arr[0]), cmp);
    }
    return 0;
}

So for input,

1 2 
3 6 
0 8 
5 4 
8 9 
5 7

Output is,

1 2
5 4
3 6
5 7
0 8
8 9

I tried but couldn't sort them according to their 2nd column. I am confused with passing array element to the comparator. Also with what to pass as the size of the element? But I guess that first 2 among below are correct.

qsort(arr, n, sizeof(arr[0]), cmp);
//qsort(arr, n, sizeof((int *)), cmp);
//qsort(arr, n, 2 * sizeof((int)), cmp);

I tried various combinations for the comparator.

Please hint out the way or perhaps an explanation.

Answer 1

The prototype for the comparison function is specified in the prototype for the qsort function:

void qsort(void *base, size_t nmemb, size_t size,
           int (*compar)(const void *, const void *));

Your comparison function must be compatible, so you can define it this way:

int cmp(const void *a, const void *b) {
    /* a and b are pointers to the array of pointers. */
    int *aa = *(int * const *)a;
    int *bb = *(int * const *)b;
    return (aa[1] > bb[1]) - (aa[1] < bb[1]);
}

Or without casts:

int cmp(const void *a, const void *b) {
    /* a and b are pointers to the array of pointers. */
    int * const *aa = a;
    int * const *bb = b;
    int ia = (*aa)[1];
    int ib = (*bb)[1];
    return (ia > ib) - (ia < ib);
}

Note that you cannot use the simplistic comparison aa[1] - bb[1] as it can overflow for large values and at best produce incorrect output.

Furthermore, you input loop is incorrect: you should nest the loops in the opposite order:

    for (int k = 0; k < n; k++) {
        for (int j = 0; j < 2; j++) {
           scanf("%d", &arr[k][j]);
        }
    }

Here is a modified version:

#include <stdio.h>
#include <stdlib.h>

int cmp(const void *a, const void *b) {
    /* a and b are pointers to the array of pointers. */
    int * const *aa = a;
    int * const *bb = b;
    int ia = (*aa)[1];
    int ib = (*bb)[1];
    return (ia > ib) - (ia < ib);
}

int main() {
    int t; // test cases
    scanf("%d", &t);
    for (int i = 0; i < t; i++) {
        int n;
        scanf("%d", &n); // size of array
        int **arr = malloc(sizeof(*arr) * n);

        for (int j = 0; j < n; j++) {
            arr[j] = malloc(sizeof(*arr[j]) * 2);
        }
        for (int k = 0; k < n; k++) {
            for (int j = 0; j < 2; j++) {
                scanf("%d", &arr[k][j]);
            }
        }
        qsort(arr, n, sizeof(arr[0]), cmp);
        for (int k = 0; k < n; k++) {
            for(int j = 0; j < 2; j++) {
                printf("%d\t", arr[k][j]);
            }
            printf("\n");
        }
        for (int j = 0; j < n; j++) {
            free(arr[j]);
        }
        free(arr);
    }
    return 0;
}

You can also simplify the code using an actual 2D array:

#include <stdio.h>
#include <stdlib.h>

int cmp(const void *a, const void *b) {
    const int *aa = a;
    const int *bb = b;
    return (aa[1] > bb[1]) - (aa[1] < bb[1]);
}

int main() {
    int t; // test cases
    scanf("%d", &t);
    for (int i = 0; i < t; i++) {
        int n;
        scanf("%d", &n); // size of array
        int (*arr)[2] = malloc(sizeof(*arr) * n);

        for (int k = 0; k < n; k++) {
            scanf("%d%d", &arr[k][0], &arr[k][1]);
        }
        qsort(arr, n, sizeof(arr[0]), cmp);
        for (int k = 0; k < n; k++) {
            printf("%d\t%d\n", arr[k][0], arr[k][1]);
        }
        free(arr);
    }
    return 0;
}