Question

Initializing a matrix as so seems to link the rows so that when one row changes, they all change:

>>> grid = [[0]*5]*5
>>> grid
[[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]]
>>> grid[2][2] = 1
>>> grid
[[0, 0, 1, 0, 0],
 [0, 0, 1, 0, 0],
 [0, 0, 1, 0, 0],
 [0, 0, 1, 0, 0],
 [0, 0, 1, 0, 0]]

How can I avoid this?

Answer 1

grid = [[0]*5 for i in range(5)]

Note: [int]*5 copies the int 5 times (but when you copy an int you just copy the value). [list]*5 copies the reference to the same list 5 times. (when you copy a list you copy the reference that points to the list in memory).