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

2 views (last 30 days)
sermet on 9 Jan 2013
example; 1 2 3 2 4 6 3 6 9 2nd 3rd
Jan on 9 Jan 2013
Please, sermet, format your messages properly. Do you see that it is confusing? There is a "? Help" button on top of the mask to input message. Please read the instructions presented there.
Posting two questions about the same problem is called "double posting" and not liked in forums. Therefore this forum offers the possibility to edit a question in case of unclear formulations.

Jan on 9 Jan 2013
And:
m(:, [2,3]) = m(:, [3,2])

Image Analyst on 9 Jan 2013
Another way:
column2 = m(:,2); % Save old column2.
m(:,2) = m(:,3); % Move col3 to col2.
m(:,3) = column2; % Make col3 the old col2
Image Analyst on 9 Jan 2013
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 on 10 Jan 2013
@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.

Amith Kamath on 9 Jan 2013
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?