#include <stdio.h>
#include <math.h>

#define MAXDATA 1000

float cor(float[], float[], int);

int main() {
  int numdata;
  float x[MAXDATA];
  float y[MAXDATA];
  float correlation;

  /* (1) The first line of the file 'test.dat' consists of a single */
  /* integer indicating the number of lines of data that follow.    */
  /* Read the integer into the variable 'numdata'.  If the value    */
  /* is greater than MAXDATA, print an error message and return.    */

  /* (2) The 'numdata' lines in 'test.dat' consist of two floating- */
  /* point number separated by a comma.  For each line, read the    */
  /* first number into the array x[] and the second number into     */
  /* the array y[].  Close the file after you have read all of      */
  /* the data.                                                      */
  
  /* Call the correlation() function - DON'T CHANGE THIS            */
  correlation = cor(x, y, numdata);

  /* (3) Open the file 'test.out' for writing.  Write the following */
  /* message to the file:                                           */
  /*       "The correlation is <float value>."                      */
  /*                               ^^^^^^                           */
  /*                   replace with value of correlation            */
  /* Close the file after you have written the output.              */

  return 0;
}

/* cor() - compute the correlation of the data in x[] and y[] */
/*   x[] - first data array                                   */
/*   y[] - second data array                                  */
/*   len - length of x[] and y[]                              */

float cor(float x[], float y[], int len) {
  float sumx = 0.0;
  float sumy = 0.0;
  float sumx2 = 0.0;
  float sumy2 = 0.0;
  float sumxy = 0.0;
  float ans, num, den;
  int i;

  for (i = 0; i < len; i++) {
    sumx += x[i];
    sumy += y[i];
    sumx2 += (x[i] * x[i]);
    sumy2 += (y[i] * y[i]);
    sumxy += (x[i] * y[i]);
  }

  num = ( len * sumxy - sumx * sumy );
  den = sqrt(len * sumx2 - sumx * sumx) * sqrt(len * sumy2 - sumy * sumy );
  ans = num / den;

  return ans;
}