averaging list of lists python column-wise

Question

I have a list of lists: something like:

data = [[240, 240, 239],         [250, 249, 237],          [242, 239, 237],         [240, 234, 233]] 

And I want to average this out like

[average_column_1, average_column_2, average_column_3] 

My piece of code is like not very elegant. It is the naive way of going thru the list, keeping the sum in seperate container and then dividing by number of elements.

I think there is a pythonic way to do this. Any suggestions? Thanks

Answer 1

Pure Python:

from __future__ import division def mean(a):     return sum(a) / len(a) a = [[240, 240, 239],      [250, 249, 237],       [242, 239, 237],      [240, 234, 233]] print map(mean, zip(*a)) 

printing

[243.0, 240.5, 236.5] 

NumPy:

a = numpy.array([[240, 240, 239],                  [250, 249, 237],                   [242, 239, 237],                  [240, 234, 233]]) print numpy.mean(a, axis=0) 

Python 3:

from statistics import mean a = [[240, 240, 239],      [250, 249, 237],       [242, 239, 237],      [240, 234, 233]] print(*map(mean, zip(*a))) 

Answer 2

data = [[240, 240, 239],         [250, 249, 237],          [242, 239, 237],         [240, 234, 233]] avg = [float(sum(col))/len(col) for col in zip(*data)] # [243.0, 240.5, 236.5] 

This works because zip(*data) will give you a list with the columns grouped, the float() call is only necessary on Python 2.x, which uses integer division unless from __future__ import division is used.