Python List Indexes

Question

I have a dual list and I am wondering what it the best way to get the indexes of the zeros in the array

board =[[1,2,0],
    [2,1,2],
    [1,1,0]]

for boxes in board:
    if 0 in boxes:
        print boxes

like this but instead I want to have return [0,2] [2,2]

if board were a NumPy array, you could use zip(*numpy.where(a == 0)). — Sven Marnach
– Sven Marnach, Commented Mar 28, 2012 at 19:43
@IgnacioVazquez-Abrams Does this matter here? I'm not sure, but I think he's always printing the whole boxes... so I think there could also be 3 zeros in, but it would still work. — evotopid
– evotopid, Commented Mar 28, 2012 at 19:45

ThiefMaster · Accepted Answer · 2012-03-28 20:32:33Z

4

Your question is very vague (what about multiple zeroes in one of the inner lists), feel free to comment if you are looking for something else:

zeroes = []
for x, box in enumerate(board):
    if 0 in box:
        zeroes.append((x, box.index(0)))
print zeroes

With your given lists, this prints

[(0, 2), (2, 2)]

A shorter, more pythonic version would be using a list comprehension like this:

zeroes = [(x, box.index(0)) for x, box in enumerate(board) if 0 in box]

edited Mar 28, 2012 at 20:32

answered Mar 28, 2012 at 19:44

ThiefMaster

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3 Comments

GeneralZero Over a year ago

I did not know about the enumerate function that will diffidently come in handy. Is there a way to condense the for if loop in to one line?

Sven Marnach Over a year ago

@GeneralZero: This can be written in one line as zeroes = [(x, box.index(0)) for x, box in enumerate(box) if 0 in box]. Note that this code (as well as the code in the above answer) will at most include one zero per row -- not sure if this is what you want.

GeneralZero Over a year ago

@SvenMarnach Thank you for pointing that out. I in fact do need more than one per row

Sven Marnach · Accepted Answer · 2012-03-28 19:45:36Z

3

You could use a list comprehension:

[(i, j) for i in range(3) for j in range(3) if board[i][j] == 0]

This will include multiple zeros per row if present.

answered Mar 28, 2012 at 19:45

Sven Marnach

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