python 2d array, print single elements inside assign loop different from print the whole array

Question

I have encounter a very strange python problem, code below. I input a grid (2d array) [[7, 8, 9], [4, 5, 6], [1, 2, 3]]

from typing import List

def test(
        height: int,
        width: int,
        grid: List[List[int]]):

    print(grid)

    refList = [[0]*width]*height
    for h in range (0, height):
        for w in range (0, width):
            if h == 0 and w == 0:
                refList[h][w] = grid[h][w]
            elif(h < 1):
                refList[h][w] = refList[h][w-1] +grid[h][w]
            elif(w < 1):
                refList[h][w] = refList[h-1][w] + grid[h][w]
            else:
                refList[h][w] = grid[h][w] + max(refList[h-1][w], refList[h][w-1])
            print(refList[h][w])
    
    print(refList)

grid = [[7,8,9],[4,5,6],[1,2,3]]
test(3,3,grid)

I expect the output [[7,15,24],[11,20,30],[12,22,33]]. as you can see below, the print function inside the loop prints the correct answers, but after the loop finished and print the whole list, it gave me this [[12, 22, 33], [12, 22, 33], [12, 22, 33]] - basically all top row.

[[7, 8, 9], [4, 5, 6], [1, 2, 3]]
7
15
24
11
20
30
12
22
33
[[12, 22, 33], [12, 22, 33], [12, 22, 33]]

What is going on here and how can I fix this?

Jérôme Richard · Accepted Answer · 2021-04-15 13:30:53Z

2

You are experimenting an issue due to shallow copies. You should use:

refList = [[0 for i in range(width)] for j in range(height)]

in replacement of your current initialisation line:

refList = [[0]*width]*height

Indeed, this last line create an array of reference to the same line-based array internally ([[0]*width]). You can check that easily use the following code:

refList = [[0]*3]*3
print(refList[0] is refList[1])  # Print True

edited Apr 15, 2021 at 13:30

answered Apr 15, 2021 at 13:25

Jérôme Richard

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