Matlab: reshape 3-dimensional array into 2-dimensional array

Question

I have a 3x3x2000 array of rotation matrices that I need to transform into a 2000x9 array.

I think I have to use a combination of permute() and reshape(), but I don't get the correct output order.

This is what I need:

First row of 3x3 array needs to be columns 1:3 in the output
Second row of 3x3 array needs to be columns 4:6 in the output
Third row of 3x3 array needs to be columns 7:9 in the output

I have tried all possible combinations of numbers 1 2 3 in the following code:

out1 = permute(input, [2 3 1]);
out2 = reshape(out1, [2000 9]);

But I always end up with the wrong order. Any tips for a Matlab newbie?

Answer 1

How about a simple for-loop?

for i=1:size(myinput,3)
    myoutput(i,:)=[myinput(1,:,i) myinput(2,:,i) myinput(3,:,i)];
    % or
    % myoutput(i,:)=reshape(myinput(:,:,i),[],9);
end

It's not simple as using permute and reshape, but it is transparent and easier for debugging. Once everything in your program runs perfectly, you can consider to rewrite such for-loops in your code...

Answer 2

You had a mix-up in your permute

a = reshape(1:9*6, 3, 3, []); 

a is a 3x3x6 matrix, each

a(:,:,i) = 9*(i-1) + [1 4 7
                      2 5 8
                      3 6 9];

So

out1 = permute(a, [3,1,2]);
out2 = reshape(out1, [], 9);

Or in one line

out3 = reshape(permute(a, [3,1,2]), [], 9);

So

out2 = out3 =

     1     2     3     4     5     6     7     8     9
    10    11    12    13    14    15    16    17    18
    19    20    21    22    23    24    25    26    27
    28    29    30    31    32    33    34    35    36
    37    38    39    40    41    42    43    44    45
    46    47    48    49    50    51    52    53    54