Create new matrix containing only subset of values from old matrix

Hi guys!

As a beginner I'm working with neural data in Matlab and I was hoping someone could help me with the following:

I have a matrix ('connectivity_matrix') of size 22 x 24 x 24 (22 dyads/subject pairs, 24 electrodes subject 1, 24 electrodes subject 2) which contains connectivity values.

Now, I want to create a new matrix ('new_connectivity_matrix') that for every dyad (first dimension of connectivity_matrix) only contains a specific subset of those connectivity values, from specific electrode combinations as indicated by the last two dimensions. Therefore, I created a binary matrix ('binary_matrix') of size 24 x 24 (24 electrodes subject 1, 24 electrodes subject 2) where the ones indicate which electrode combinations I want to keep in my new matrix. So to be clear, this binary matrix indicates which values (electrodes combinations) I want to keep in my new matrix voor every dyad.

I was thinking of looping over dyads (first dimension), and for every dyad elementwise multiple the 'binary_matrix' with the 'connectivity_matrix' so that for every dyad, the same electrode combinations as indicated by ones will be in my new matrix:

connectivity_matrix; % this is the connectivity matrix containing all dyads (22x24x24)
binary_matrix; % this is the binary matrix indicating which electrode combinations I want to keep in my new matrix (24x24)

new_connectivity_matrix = zeros(22,24,24); % initialize new matrix

for dyad = 1:22
    new_connectivity_matrix = connectivity_matrix(dyad,:,:) .* binary_matrix(:,:)
end
  

I know something is not right.. The size of 'new_connectivity_matrix' for example is 24x24x24, while I assumed it should be 22x24x24?

I hope my explaination is a bit clear and someone can help me out! Thank you in advance!

Answer 1

[Scott MacKenzie]

For the matrix multiplication, use squeeze so both matricies are 24x24, then assign the result to the corresponding 1st dimension of the new matrix:

for dyad = 1:22
    new_connectivity_matrix(dyad,:,:) = squeeze(connectivity_matrix(dyad,:,:)) .* binary_matrix(:,:)
end

[Alix Weidema]

Thanks a lot Scott!