What is wrong with this python function from “Programming Collective Intelligence”?

Question

This is the function in question. It calculates the Pearson correlation coefficient for p1 and p2, which is supposed to be a number between -1 and 1.

When I use this

Answer 1

It looks like you may be unexpectedly using integer division. I made the following change and your function returned 1.0:

num=pSum-(1.0*sum1*sum2/n)
den=sqrt((sum1Sq-1.0*pow(sum1,2)/n)*(sum2Sq-1.0*pow(sum2,2)/n))

See PEP 238 for more information on the division operator in Python. An alternate way of fixing your above code is:

from __future__ import division