Extract 2D array from 3D array using logical index

I have a PxMxN array that I want to convert in a PxK 2D array. K has to be obtained from a logical matrix MxN. Consequently, numel(K)<=numel(MxN). Can anyone help me? Thanks.

Answer 1

[Stephen23]

"I have a PxMxN array that I want to convert in a PxK 2D array. K has to be obtained from a logical matrix MxN. Consequently, numel(K)<=numel(MxN). Can anyone help me?"

Just use the indexing and then RESHAPE (which does not move any data in memory so is very efficient):

format compact
A = randi(9,5,4,3)

A = 
A(:,:,1) =
     8     1     6     4
     4     5     5     3
     3     8     4     7
     3     3     6     4
     6     1     8     7
A(:,:,2) =
     6     3     4     5
     1     5     9     7
     6     8     8     1
     2     4     3     9
     6     9     7     7
A(:,:,3) =
     2     4     8     7
     9     5     1     9
     9     4     1     4
     5     5     9     8
     3     7     2     2

X = randi(0:1,4,3);
X = logical(X)

X = 4x3 logical array
   0   1   1
   0   0   0
   1   1   0
   0   1   0

B = A(:,X); % easy indexing
B = reshape(B,size(A,1),[])

B = 5×5

6 6 4 5 2 5 1 9 7 9 4 6 8 1 9 6 2 3 9 5 8 6 7 7 3

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Checking the first of the index values:

[R,C] = find(X,1,'first')

R = 3

C = 1

A(:,R,C)

ans = 5×1

6 5 4 6 8

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This works because MATLAB applies the final index to all trailing dimensions:

https://blogs.mathworks.com/loren/2006/08/09/essence-of-indexing/

An interesting side-effect of this is that linear indexing is really just subscript indexing with one index.