python nested list value change not working

Question

for i in range(len(arr)):
     temps = [[0]*9]*3   
# temp = [[0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 
0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0]]

when tries

temps[0][0] = 4

gives this output

[[4, 0, 0, 0, 0, 0, 0, 0, 0], [4, 0, 0, 0, 0, 0, 0, 
0, 0], [4, 0, 0, 0, 0, 0, 0, 0, 0]]

tries to change the value of temp[0][0] it changes all temp[0][0] temp[0][1] temp[0][2] value

How did you initialize the `temp` list? See here: https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly — Andrej Kesely, Oct 07 '20 at 08:50

score 1 · Answer 1 · answered Oct 07 '20 at 09:01

1

Christian is right. [t] * 3 is equivalent to [t, t, t]

You can solve it by doing this:

for i in range(len(arr)):
     temps = [[0]*9 for _ in range(3)]

answered Oct 07 '20 at 09:01

Jasmijn

you don't need the forloop, this just initializes temps len(arr) number of times. – Christian Sloper Oct 07 '20 at 09:02
1

@ChristianSloper you're right but I assume this is only part of the code, considering the `range(len(arr))`. – Jasmijn Oct 07 '20 at 09:10

1 Answers1