What is the correlation of [1,2,3] and [1,5,7] to 8 decimal digits?

Question

This stackoverflow post describes computing a Pearson correlation of [1,2,3] and [1,5,7] in several different ways in Python. The most straightforward implementation from the definition in Wikipedia comes up with

0.973328526785

while Excel, R, NumPy, an online calculator, and a different Python implementation (involving what looks like a more numerically unstable calculation to me) come up with

0.981980506

I am just curious to know what you think.

Answer 1

(originally written as a comment)

Beware of the type of the variables!

You have encountered an int/float problem. In sum(x) / len(x) you divide ints, not floats. So sum([1,5,7]) / len([1,5,7]) = 13 / 3 = 4, according to the integer division rules (whereas you want 13. / 3. = 4.33...). To fix it rewrite this line as float(sum(x)) / float(len(x)) (one float suffices, as Python converts it automatically).