How to look at only the 3rd value in all lists in a list

Question

I have a list of lists and I want to be able to refer to the 1st, 2nd, 3rd, etc. column in a list of lists. Here is my code for the list:

matrix = [
    [0, 0, 0, 5, 0, 0, 0, 0, 6],
    [8, 0, 0, 0, 4, 7, 5, 0, 3],
    [0, 5, 0, 0, 0, 3, 0, 0, 0],
    [0, 7, 0, 8, 0, 0, 0, 0, 9],
    [0, 0, 0, 0, 1, 0, 0, 0, 0],
    [9, 0, 0, 0, 0, 4, 0, 2, 0],
    [0, 0, 0, 9, 0, 0, 0, 1, 0],
    [7, 0, 8, 3, 2, 0, 0, 0, 5],
    [3, 0, 0, 0, 0, 8, 0, 0, 0],
    ]

I want to be able to say something like:

matrix = [
    [0, 0, 0, 5, 0, 0, 0, 0, 6],
    [8, 0, 0, 0, 4, 7, 5, 0, 3],
    [0, 5, 0, 0, 0, 3, 0, 0, 0],
    [0, 7, 0, 8, 0, 0, 0, 0, 9],
    [0, 0, 0, 0, 1, 0, 0, 0, 0],
    [9, 0, 0, 0, 0, 4, 0, 2, 0],
    [0, 0, 0, 9, 0, 0, 0, 1, 0],
    [7, 0, 8, 3, 2, 0, 0, 0, 5],
    [3, 0, 0, 0, 0, 8, 0, 0, 0],
    ]
if (The fourth column in this matrix does not have any 1's in it):
    (then do something)

I want to know what the python syntax would be for the stuff in parenthesis.

Answer 1

The standard way to perform what you asked is to do a list comprehension

if (The fourth column in this matrix does not have any 1's in it):

translates in:

>>>if not any([1 == row[3] for row in matrix])

However, depending on how often you need to perform this operation, how big is your matrix, etc... you might wish to look into numpy as it is easier (and remarkably faster) to address columns. An example:

>>> import numpy as np
>>> matrix = np.random.randint(0, 10, (5, 5))
>>> matrix
array([[3, 0, 9, 9, 3],
       [5, 7, 7, 7, 6],
       [5, 4, 6, 2, 2],
       [1, 3, 5, 0, 5],
       [3, 9, 7, 8, 6]])
>>> matrix[..., 3]  #fourth column
array([9, 7, 2, 0, 8])