Python Array Cleanup Issues

Question

I have a matrix-style array, that (hypothetically) looks like this:

mat = [[0,2,0,0],[0,0,0,0],[0,0,0,0],[0,0,0,0]]
imat = mat
for i in xrange(4):
    for j in xrange(4):
        imat[j][i] = mat[i][j]
for i in xrange(4):
    for j in xrange(4):
        imat[j][i] = mat[i][j]

The code basically switches the row/column from "mat" to "imat".

The results: mat: [[0, 2, 0, 0], [2, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]] imat: [[0, 2, 0, 0], [2, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

Could anyone tell me why the array items are duplicating like this?
Also, if there is a more efficient way to do this operation, that would also be appreciated.

`imat` and `mat` both point to the same object. – Ashwini Chaudhary May 10 '14 at 22:18 — Ashwini Chaudhary, May 10 '14 at 22:18
`imat = map(list, zip(*mat))` is the cleanest way IMO. – Ashwini Chaudhary May 10 '14 at 22:22 — Ashwini Chaudhary, May 10 '14 at 22:22

Óscar López · Accepted Answer · 2014-05-10T22:25:12.143

1

The problem is in this line:

imat = mat

Instead you should do this, to allocate a new 4x4 matrix at the beginning:

imat = [[0]*4 for _ in xrange(4)]

What's happening is that you didn't initialize imat correctly, you only assigned a reference to mat so both objects are one and the same, so no modification is being performed. Also, a much simpler alternative for transposing a matrix would be:

imat = [list(m) for m in zip(*mat)]

edited May 10 '14 at 22:25

answered May 10 '14 at 22:18

Óscar López

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Drive-by down voters: care to comment? what's wrong with my answer? – Óscar López May 10 '14 at 22:21
Strange. I don't see anything wrong with it. Was there a first version of lower quality, hidden by the 5-minute edit window? – user2357112 May 10 '14 at 22:25
@user2357112 nope, I posted the `imat = [[0]*4 for _ in xrange(4)]` part from the beginning, but expanded on the explanation later – Óscar López May 10 '14 at 22:26
@user2357112 Yes, the original was a "try this" answer with no explanation. – juanchopanza May 10 '14 at 22:28
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Thanks. The `imat = [[0]*4...` works really well. You've found the problem. It now works. – bcdan May 10 '14 at 22:38

Lie Ryan · Answer 2 · 2014-05-10T22:30:09.250

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Also, if there is a more efficient way to do this operation, that would also be appreciated.

Yes, it's called a matrix transpose operation, which in Python is done using the builtin function zip() with the *-unpacking:

imat = zip(*mat)

As to why your current code doesn't work, @Óscar López has it right, doing imat = mat does not create a new copy of the matrix.

edited May 10 '14 at 22:30

answered May 10 '14 at 22:21

Lie Ryan

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Python Array Cleanup Issues

2 Answers2