C - Sort float array while keeping track of indices

Question

I have an array of 3 floating point values:

float norms[3];

norms[0] = 0.4;
norms[1] = 3.2;
norms[2] = 1.7;

I want to sort this array in descending order while keeping track of the original indexes of the values in the array.

In other words, given the array norms[] = {0.4, 3.2, 1.7} with corresponding indices {0, 1, 2}, I basically want to obtain an array of corresponding ints that reflects the original positions of the float values in norms[] following a descending sort. In this case it would be {1, 2, 0}.

What is the best/cleanest way to achieve this?

Answer 1

Use a structure to store the value as well as index and then sort according to value.

struct str
{
    float value;
    int index;
};

int cmp(const void *a, const void *b)
{
    struct str *a1 = (struct str *)a;
    struct str *a2 = (struct str *)b;
    if ((*a1).value > (*a2).value)
        return -1;
    else if ((*a1).value < (*a2).value)
        return 1;
    else
        return 0;
}

int main()
{
    float arr[3] = {0.4, 3.12, 1.7};
    struct str objects[3];
    for (int i = 0; i < 3; i++)
    {
        objects[i].value = arr[i];
        objects[i].index = i;
    }
    //sort objects array according to value maybe using qsort
    qsort(objects, 3, sizeof(objects[0]), cmp);
    for (int i = 0; i < 3; i++)
        printf("%d ", objects[i].index); //will give 1 2 0
    // your code goes here
    return 0;
}