Question

I’m trying to sort an array A whose elements are indexes. The indexes refer to another array B whose value will determine the order of A. So, I would like to sort A such that B[ A[i] ] is increasing.

For example:

A = [0, 1, 4, 5, 7]
B = [5, 3, 8, 2, 2, 7, 1, 6, 3, 9]

Sorted A would be

A' = [ 7, 4, 1, 0, 5 ]

Is this possible with C’s built-in sort, or am I going to have to write my own implementation?

EDIT: These arrays are local function variables.

Answer 1

If you want to use qsort, the best thing to-do would be to re-wrap the indexes in A and the values in B into a struct, and then make a comparator based on a new array that struct. For instance:

typedef struct
{
    int index_from_A;
    int value_from_B;
} index_value_wrapper;

index_value_wrapper index_wrapper_array[5];

for (int i=0; i < 5; i++)
{
    index_wrapper_array[i].index_from_A = A[i];
    index_wrapper_array[i].value_from_B = B[A[i]];
}

int comparitor (const void* lhs, const void* rhs)
{
    return (lhs.value_from_B - rhs.value_from_B);
}

Now you can run qsort on the struct array and from there you can extract the proper sorted sequence you desired for the original array A without having to use a custom sorting function.