Assignment to a 2d list

Question

I was having a go at making a simple machine learning program so I could understand more about AI, but ran into a problem. My program has a 2D list (101 lists of 3 Boolean values (303 total)). It does several loops, and has to set 1, or 2 values from one of the sub lists to True in each loop. When running my program, all values changed to true far too quickly. At first, I thought this might be due to bad luck with random(), but then I saw that all the values changed after only 90 to 100 loops, which should be impossible.

I decided to experiment in the shell.

>>> Choises=[[False]*3]*101
>>> Choises[1][1]=True
>>> Choises
[[False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False], [False, True, False]]

Why on earth did it change the 2^nd value of each list, rather than just the 2^nd value of just the 2^nd one?

thyago stall · Accepted Answer · 2018-05-22T20:11:38.443

2

What happens here is that when you create your list, you are multiplying the same reference over and over. So:

choices = [[False] * 3] * 101

A better way to achieving what you want is:

choices = [[False for _ in range(3)] for _ in range(101)]
choices[1][1] = True

edited May 22 '18 at 20:11

answered May 22 '18 at 19:58

thyago stall

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Best to stay out of the habit of using `[...] * n` altogether if you don't actually need to; `[[False for _ in range(3)] for _ in range(101)]`. – chepner May 22 '18 at 20:07
@chepner Why is this? – Programmer S May 23 '18 at 20:59

Assignment to a 2d list

1 Answers1