Reordering columns in a matrix

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)?

[Jan]

More efficient than -1*ones:

B = -ones(size(A));

or

B = repmat(-1, size(A));

Answer 1

[the cyclist]

I guess you could do

 C = A(:,[2 1 3]).*B(:,[2 1 3]);

But why?

Answer 2

[Omar Mian]

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.

Answer 3

[the cyclist]

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

http://www.mathworks.com/matlabcentral/answers/1325-what-is-missing-from-matlab

Answer 4

[Fangjun Jiang]

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]}))

Answer 5

[Andrei Bobrov]

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

[Fangjun Jiang]

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?