Updating 2D Array Values in Python - Updating whole column wrong? [duplicate]

Question

I am trying to create a 2D array as such and just update single values at a time, shown here:

M = [[0]*3]*3
M[0][0] = 3
print(M)

which is returning the following:

[[3, 0 , 0], [3, 0, 0], [3, 0, 0]]

Anyone have an idea of what I've done wrong?

Answer 1

What your first line is doing is creating one inner length 3 list, and adding three references of it to your outer list M. You must declare each internal list independently if you want them to be independent lists.

The following is different in that it creates 3 separate instances of inner length 3 lists:

M = [[0]*3 for _ in range(3)]
M[0][0] = 3
print(M)

OUTPUT

[[3, 0, 0], [0, 0, 0], [0, 0, 0]]