swap columns of a matrix

Rasmus 님이 질문을 제출함. 25 Feb 2014

최근 활동 Helen Joan 님이 댓글을 추가함. 30 Apr 2019

Thomas 님이 답변을 채택함.

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Hello guys/girls

How do i swap columns??

I have this

k_minus =

   -46   -43   -26   -14     7    19    11    32    39    45    45
     0    -4    -7    -7   -44   -44    -7    -7   -15   -15     0

and I want the columns to be in opposite order - How do I do this? and can do this in one go?

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Answer by Thomas on 25 Feb 2014

Edited by Thomas on 25 Feb 2014

Accepted Answer

You could also use

out= fliplr(k_minus)

This flips the matrix left-right http://www.mathworks.com/help/matlab/ref/fliplr.html

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Rasmus on 25 Feb 2014

That is actually easier! :)

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Answer by Mischa Kim on 25 Feb 2014

Edited by Mischa Kim on 25 Feb 2014

Use

k_minus_rev = k_minus(:, [length(k_minus(1,:)):-1:1])

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Mischa Kim on 25 Feb 2014

Basically, when you do

k_minus(:,1)

you extract the sub-matrix consisting of all rows in the first column. With

k_minus(:,[2,1])

you extract all rows of the second, and the first columns, in that order. You can now generalize the last bit

[length(k_minus(1,:)):-1:1]

to extract all columns ( length(k_minus(1,:)) ), placing the last column first, and the first column last, in descending order ( -1 ).

Jos (10584) on 25 Feb 2014

You do not need the length statement by using the keyword END:

MyMatrix_withReversedColumns = MyMatrix(:,end:-1:1)

but I do suggest you stick to FLIPLR. It is the same, but much easier to read!

kk on 2 Apr 2019

Thank you for this answer! It has helped me "merge" two matrices, i.e. creating a matrix consisting of the first column of matrix A, first column of matrix B, second column of matrix A, second column of matrix B, etc.

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Answer by rishabh gupta on 12 Jan 2018

you can also use: k_minus_rev = k_minus(:, [length(k_minus):-1:1])

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Stephen Cobeldick on 12 Jan 2018

The square brackets are totally superfluous, and using end is simpler than using length (of course length should be avoided generally because its output changes depending on the input array size: better to use numel or size with a specific dimension).

So a better (simpler, clearer, less buggy) is exactly as Jos already gave four years ago:

k_minus(:,end:-1:1)

Helen Joan on 30 Apr 2019

is it possible to do this with a loop?

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swap columns of a matrix

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