Reversing Columns in a Matrix

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?

Answer 1

[Sven]

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?

[David]

So how would I implement this if I need to run a test on all permutations of column flips? For example I need to run the test for columns 2:3, 2:4, 2:5, 2:6, 3:4, 3:5, etc. flipped.

[Image Analyst]

Just use variables for the starting and stopping element. So as soon as you've generated a pair (let's call them s1 and s2) you'd do

b = [a(1:s1-1) fliplr(a(s1:s2)) a(s2+1:end)];

[Sven]

I've updated the answer based on this new request for what you want to do. Does it answer the question?