Swap/Transpose matrix columns

Question

I have a 2d array like this:

m = [[1, 2, 3],
     [4, 5, 6],
     [7, 8, 9]]

And I want to swap the first and second columns, which would look like this:

[2, 1, 3]
[5, 4, 6]
[8, 7, 9]

I've looked around, but all I can find is stuff about turning rows into columns. I tried the zip function for example, but again it just does this:

[1, 4, 7]
[2, 5, 8]
[3, 6, 9]

Anyone know how to do this?

Using numpy: https://stackoverflow.com/questions/24507550/swapping-columns-with-numpy-arrays — Cory Kramer, Jan 27 '21 at 18:00

score 2 · Answer 1 · answered Jan 27 '21 at 18:05

2

I think if you just want the first and second columns swapped you should change the code to be something like this:

for i in m:
    i[1],i[0] = i[0],i[1]

answered Jan 27 '21 at 18:05

rachelyw

This works fine - each row is just a sublist, and want to swap first, second item - which is `[1]` and [0]`. However, it is more nature to write this way: for i in m: i[0], i[1] = i[1], i[0] – Daniel Hao Jan 28 '21 at 12:36

Epsi95 · Accepted Answer · 2021-01-28T12:37:48.247

0

for i in m:
    i[0],i[1] = i[1],i[0]

output

[[2, 1, 3], 
[5, 4, 6], 
[8, 7, 9]]

edited Jan 28 '21 at 12:37

answered Jan 27 '21 at 17:51

Epsi95

This does not work. It will get you - >>> m [[3, 2, 1], [6, 5, 4], [9, 8, 7]] – Daniel Hao Jan 28 '21 at 12:34
1

@thank you for correcting me, I read the question as the first column and last column. Again thank you. – Epsi95 Jan 28 '21 at 12:38

2 Answers2