Question

I am reading the book Programming Collective Intelligence, What exactly the following piece of python code do?

  # Add up the squares of all the differences 
  sum_of_squares=sum([pow(prefs[person1][item]-prefs[person2][item],2) 
                      for item in prefs[person1] if item in prefs[person2]]) 

I am trying to play with the examples in Java.

Prefs is a map of person to movie ratings, movie ratings is another map of names to ratings.

Answer 1

First it constructs a list containing the results from:

for each item in prefs for person1:
    if that is also an item in the prefs for person2:
        find the difference between the number of prefs for that item for the two people
        and square it (Math.pow(x,2) is "x squared")

Then it adds those up.