Accessing a list given a condition on another list in Python

Question

I want to access the elements of a list given a condition on another list. Normally, I would do this by using NumPy, but the requirements I have to follow state that I have to stick to Python Standard Library. An example of my problem is the following:

x = [1, 7, 11, 8, 13, 2]
y = [0,3,0,5,2]
#NumPy notation
z = x[y==0]

I have come up with a solution by using a list comprehension:

z = [x[i] for i in xrange(len(y)) if y[i] == 0]

However, it is quite slower in comparison to my implementation in NumPy. Is there any better way to address this?

EDIT: I have not mentioned but my requirements include the use of python 2

Answer 1

You can zip() the items together:

try:
    # iterator zip from Python 3
    from future_builtins import zip
except ImportError:
    # this *is* Python 3
    pass

z = [xval for xval, yval in zip(x, y) if yval == 0]

This also avoids building a list for the zip() even when using Python 2.

Answer 2

I'd personally use iterators and zip (if one list is shorter, it will be quicker)

That way I avoid the double access to elements and I don't have exception if a list is shorter than the other (you should use in xrange(min(len(x),len(y)) )

[a for (a,b) in zip(x,y) if b==0]

Note that zip is iterator for python 3, not for python 2

I haven't benchmarked though