C qsort() with dynamic n by 2 multi-dimensional array

Question

First, I defined a dynamic array with 2 columns and 10 row. The integer number is set to 10 here just for example.

int** array;
int number = 10;         


        

Answer 1

sample code

#include 
#include 

int compare ( const void *pa, const void *pb ) {
    const int *a = *(const int **)pa;
    const int *b = *(const int **)pb;
    if(a[0] == b[0])
        return a[1] - b[1];
    else
        return a[0] - b[0];
}
/*
#define NUMCMP(x,y) (((x) < (y)) ? -1 : ((x) > (y)) ? 1 : 0)

int compare ( const void *pa, const void *pb ) {
    const int (*a)[2] = *(const int (**)[2])pa;
    const int (*b)[2] = *(const int (**)[2])pb;
    int tmp;
    if((tmp=NUMCMP((*a)[0], (*b)[0]))==0)
        return NUMCMP((*a)[1], (*b)[1]);
    else
        return tmp;
}
*/    
int main(void){
    int **array;
    int number = 10;
    int i;

    array = malloc(number * sizeof(int*));
    for (i = 0; i < number; i++){
        array[i] = malloc(2 * sizeof(int));
        array[i][0] = rand()%20;
        array[i][1] = rand()%20;
    }
    for(i = 0;i < number;++i)
        printf("%2d, %2d\n", array[i][0], array[i][1]);

    printf("\n");

    qsort(array, number, sizeof array[0], compare);

    for(i = 0;i < number;++i)
        printf("%2d, %2d\n", array[i][0], array[i][1]);

    return 0;
}

what *(const int **)pa

array = {(int *), (int *), (int *), ... , (int *) }

qsort need each element address for element (for swap, etc. Because the size and number and start address of the element since only the given information).

E.g &(int *), &(int *)

so (int **) pass to function compare.

call compare(int **, int **) &(int*) meant at arg int**

compare function prototypeis cmp(const void*, const void*)

cast (const int**)pa is cast to passed original pointer.

*((const int **)pa) is dereference original element pointer(int*)