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Reordering columns in a matrix

Asked by Omar Mian on 30 Dec 2011
Hello,
I want to multiply 2 matrices elemntwise and swap columns 1 and 2 in the result:
A = [1 2 3; 1 2 3; 1 2 3; 1 2 3];
B = -1*ones(size(A));
AB = A.*B;
C = AB(:,[2 1 3]);
I am using 2 steps to do this (lines 3 & 4). How do I do this in a single step (single line of code)?

  1 Comment

Jan
on 30 Dec 2011
More efficient than -1*ones:
B = -ones(size(A));
or
B = repmat(-1, size(A));

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5 Answers

Answer by the cyclist
on 30 Dec 2011

I guess you could do
C = A(:,[2 1 3]).*B(:,[2 1 3]);
But why?

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Answer by Omar Mian on 30 Dec 2011

Thanks for the suggestions. I should have been more explicit in my question. I want to do the arithmetic before rearranging because in actual use, the equation will involve more variables than used in the example. I tried the following:
C = (A.*B)(:,[2 1 3])
C = [A.*B](:,[2 1 3])
Neither of these work, but I don't understand why that syntax is inappropriate.
"But why?"
Largely curiosity. I want to know if I can avoid the intermediate step.

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Answer by the cyclist
on 30 Dec 2011

You cannot index results in that way. See, for example, the accepted answer in this thread about features that are "missing" in MATLAB:

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Answer by Fangjun Jiang on 30 Dec 2011

If you have to do this, you can use subsref()
a=magic(3)
b=subsref(a,substruct('()', {':',[2,1,3]}))
In your case, it will be
C = subsref(A.*B, substruct('()', {':',[2,1,3]}))

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Answer by Andrei Bobrov
on 30 Dec 2011

circshift(fliplr(A.*B),[0 -1])

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What a surprise answer, andrei! But I would still go with the subsref() function. After all, it is the function format of the re-arrangement. To challenge you, what if A.*B is a 4x4 or 5x5 matrix but I want to swap the first and second column?

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