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

int compare (const void * a, const void * b)
{
    if (*(double *)a > *(double *)b) return 1;
    if (*(double *)a < *(double *)b ) return -1;

    return 0;
}

int main()
{
    double X[] = { 62.0,53.0,51.0,25.0,79.2,23.7,47.9,24.1 };
    double Y[] = { 52,64.5,39.9,34,21.5,37,39,65.8 };
    const int LENGTH=8;
    // copy arrays
    double origX[LENGTH];
    double origY[LENGTH];
    for (int i=0; i<LENGTH; i++)
    {
        origX[i]=X[i];
        origY[i]=Y[i];
    }
    qsort(X, LENGTH, sizeof(double), compare);
    qsort(Y, LENGTH, sizeof(double), compare);
    printf("Sorted data\n");
    for (int i=0; i<LENGTH; i++)
        printf("%d %f %f\n",i, X[i], Y[i]);
    // replace values by ranks
    for (int i=0; i<LENGTH; i++)
    {
        double val = origX[i];
        for (int j=0; j<LENGTH; j++)
        {
            if (X[j]==val)
            {
                origX[i]=j;
                break;
            }
        }
        val = origY[i];
        for (int j=0; j<LENGTH; j++)
        {
            if (Y[j]==val)
            {
                origY[i]=j;
                break;
            }
        }
    }
    printf("Ranks\n");
    for (int i=0; i<LENGTH; i++)
        printf("%d %f %f \n",i, origX[i], origY[i]);
    // form sum of difference in rank, squared
    int sumD2=0;
    for (int i=0; i<LENGTH; i++)
    {
        int d = (int)(origX[i]-origY[i]);
        sumD2+=d*d;
    }
    // r = 1 - 6 sunD2 / (n^3-n)
    double r = 1 - 6.0*sumD2/ (LENGTH*(LENGTH*LENGTH-LENGTH));
    printf("Pearson's rank = %f\n", r);

    return 0;
}