I wanna replacing 2nd and 3rd column to each other. How can I write a command?

example; 1 2 3 2 4 6 3 6 9 2nd 3rd

[Jan]

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Answer 1

[Jan]

And:

m(:, [2,3]) = m(:, [3,2])

Answer 2

[Image Analyst]

Another way:

column2 = m(:,2); % Save old column2.
m(:,2) = m(:,3); % Move col3 to col2.
m(:,3) = column2; % Make col3 the old col2

[Jan]

Do you believe, that this is more efficient than my idea? I ask for "believing", not a measurement.

[Daniel Shub]

I am going to go with yes (but it is a guess). For m(:, [2,3]) = m(:, [3,2]) I think MATLAB would allocate a brand new n x m array while IA method would allocate a N x 1 array and reuse/modify the n x m array. That said, I would have gone with m(:, [2,3]) = m(:, [3,2]).

[Image Analyst]

I have no idea. For things this tiny I usually don't worry about it. I went with a typical "swap" method. Jan, yours is more MATLAB-ish. For larger arrays, I might time them to compare speeds.

[Jan]

@Image Analyst and Daniel: I believe that Matlab does not allocate an [n x 2] array for the right hand side m(:, [3,2]), but does something like Bruno did in his FEX: MinMaxSelection with the inplace shared subarray tricks. I will perform some tests with large arrays.

Answer 3

[Amith Kamath]

Assuming I understand the question right, here is an example of how it could be done:

x = magic(5);    % a random matrix for illustration.
l = 1:size(x,2); % get the number of columns of x.
l(2:3) = [3 2];  % setup index replacement.
y = x(:,l);      % y will have the same elements as x, with rows 2 and 3 exchanged.

helps?