MATLAB: Reversing Columns in a Matrix

matrix manipulation

I need to reverse the order of specific columns in a vector. For example:
a = [1 2 3 4 5 6]
Reverse columns 2:5
a = [1 5 4 3 2 6]
How do I do this?

Best Answer

Use fliplr to flip a matrix left-to-right:
a = [1 2 3 4 5 6]
b = [a(1) fliplr(a(2:5)) a(6)]
See also: flipud (flips it up-to-down) or flipdim (flips along a specified dimension).
(Edit based on your comments below)
So if you want to flip every permutation of elements (2:3, 2:4, 2:5, 2:6, 3:4, etc), then you can also use nchoosek as follows:
a = [1 2 3 4 5 6];
twoInds = nchoosek(1:length(a), 2);
numPerms = size(twoInds,1);
all_A = repmat(a, numPerms, 1);
for i = 1:numPerms
   colNos = twoInds(i,1):twoInds(i,2);
   all_A(i, colNos) = flipdim(a(colNos),2);
end
So now you have your original "a" variable, plus a matrix "all_A", where every row has a different piece of "a" flipped, left to right.
Was that what you were looking for?
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