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: stackoverflow.com/questions/240178/… — Andrej Kesely
– Andrej Kesely, Commented Oct 7, 2020 at 8:50

Jasmijn · Accepted Answer · 2020-10-07 09:01:35Z

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 7, 2020 at 9:01

Jasmijn

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2 Comments

you don't need the forloop, this just initializes temps len(arr) number of times.

@ChristianSloper you're right but I assume this is only part of the code, considering the range(len(arr)).