Replace Value in Only One Row?

Question

I have a list of two rows, and am just trying to replace one variable in one row, but it's changing the variable in both rows. Below is the code:

rowOfZeros_4cols = []

for i in range(0,4):
    rowOfZeros_4cols.append(0.)

twoRows = [rowOfZeros_4cols, rowOfZeros_4cols]

mat_Zeros = twoRows

mat_Zeros[0][2] = 1.

The output looks like:

[[0.0, 0.0, 1.0, 0.0], [0.0, 0.0, 1.0, 0.0]]

When I want it to look like:

[[0.0, 0.0, 1.0, 0.0], [0.0, 0.0, 0.0, 0.0]]

What am I doing wrong? This seems like it should be pretty straight forward.

Thanks in advance, Eric

Because `twoRows` is a `list` containing **the same list twice**. — juanpa.arrivillaga, Feb 13 '17 at 21:03
Possible duplicate of [List of lists changes reflected across sublists unexpectedly](http://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly) — juanpa.arrivillaga, Feb 13 '17 at 21:04
Actually, more of a duplicate of [this one](http://stackoverflow.com/questions/6360286/why-does-foo-appendbar-affect-all-elements-in-a-list-of-lists). — juanpa.arrivillaga, Feb 13 '17 at 21:05

score 0 · Answer 1 · answered Feb 13 '17 at 21:04

Here's the culprit:

twoRows = [rowOfZeros_4cols, rowOfZeros_4cols]

The two references to rowOfZeros_4cols are not two separate lists. That are the same object, referenced twice.

If you change it, it looks like "both rows" have changed. Instead, you're always changing the same row, only referring to it twice.

Generate a fresh row every time:

twoRows = [[0] * 4, [0] * 4]  # two independent lists of 4 zeros.

Replace Value in Only One Row?

1 Answers1