Problems with indexing in multidimensional array

Hi Community!

I have trouble understanding how matlab handles dimensions if I loop though one of the inner dimensions and try to build a result array which has the same size as the original one.

I have written an example:

clear test;
clear test2;

test = ones(4,1,1,3,3,4);
size(test)

for ii = 1:size(test,4)
    test2(:,:,:,ii,:,:) = ii*test(:,:,:,ii,:,:);
end

size(test2)

The size of test changes from 4,1,1,3,3,4 to test2 size of 4,3,4,3. My intention was to take the original array and multiply every element where the 4.dimension is 1 by 1, where it is 2 by 2 and so on. My expected result array test2 would have the exact same dimensions as test.

The multiplication by ii is just an example. In my original code I do a more complex calculation which I can't substitute by a matrix operation.

What am I doing wrong? What's the correct way to achieve my goal?

Thanks in advance folks!

[Stephen23]

"What's the correct way to achieve my goal?"

Preallocate the array:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[Guillaume]

Well, to see the problem, simply do:

clear test;
clear test2;

test = ones(4,1,1,3,3,4);
test2(:, :, :, 1, :, :) = test(:, :, :, 1, :, :);
size(test2)

The reason is that you're assigning something 4x1x1x1x3x4 to something that does not exists (test2). So matlab creates a matrix big enough to hold whatever your assigning, ignoring the singular dimensions. So you get a 4x3x4 matrix.

To solve this you need to tell matlab what the size of test2 is supposed to be, before you start the loop, so:

test2 = zeros(size(test));
for ...

and matlab won't have to guess (and get it wrong) what shape test2 is supposed to be. Other benefits of this:

• faster code. With your code, the size of test2 increases at each step of the loop, so matlab has to reallocate a new array at each step, copy the old one over and add the new elements
• no need for clear anymore, since if the variables already exist, you're replacing them.

Note that if you'd written your code in the editor, you'd have seen a wiggly line under test2 in the loop telling you that test2 changes size at each iteration and to consider preallocation.

[Stephen23]

+1 for a clear explanation, and for pointing out that that editor gives useful advice.

[Michael]

Thanks a lot. I already knew that my method is bad in terms of performance. But I did not see the problem with the singleton dimension. As you wrote it creates the 4x3x4 matrix in the first loop run and in the second run the test2(:,:,:,ii,:,:) command adds another dimension so that I end up with 4x3x4x3.

Thanks for the quick reply!