Nested loop indexing malfunction

Asked Dec 30 '22 at 17:08

Active Dec 30 '22 at 17:08

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I am writing a code to multiply two matrices, using the equation c[i][j] = (0<k<n)Sigma(a[i][k]*b[k][j]) But in first iteration, my final matrix goes from

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

[
    [2,0,0]
    [2,0,0]
]

That is an error because according to my code, only the [0][0]th element should be getting changed to 2, not [1][0]th element too. This is a bit hard to explain over text. Here is my code snippet which doesn't work as meant to

scaledMatrix = [[0]*3]*n

for i in range(n):
    for j in range(3):
        sum_of_elements = 0
        for k in range(n):
            sum_of_elements += coordinateMatrix[i][k]*otherMatrix[k][j]
        scaledMatrix[i][j] = sum_of_elements

asked Dec 30 '22 at 17:08

kiddo42069

Here is my full code: https://pastebin.com/MMkUz1zK – kiddo42069 Dec 30 '22 at 17:10
My inputs are: 2 1 2 3 4 2 3 – kiddo42069 Dec 30 '22 at 17:10
`scaledMatrix = [[0]*3]*n` This is the problem. When you multiply a list in this way, you get multiple identical references to _the same inner lists_. – John Gordon Dec 30 '22 at 17:11
And my incorrect output is [[6,12,0],[6,12,0]] Correct output is [[2,6,0],[6,12,0]] – kiddo42069 Dec 30 '22 at 17:12
1

Yes, as John says it's `scaledMatrix = [[0]*3]*n` that is the problem. Change it to `scaledMatrix = [[0 for _ in range(3)] for _ in range n]` – Matt Pitkin Dec 30 '22 at 17:12
To read more about why see the discussion at https://bugs.python.org/issue27135 and https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly – Matt Pitkin Dec 30 '22 at 17:16

Nested loop indexing malfunction

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