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matlabprojectionsubmatrix

MATLAB submatrix

MATLAB question:

I have an array A(2,2,2) that is three-dimensional. I would like to define a 2x2 array as a subarray of A, as follows:

B = A(1,:,:).

That is, we are simply projecting on the first component. But matlab will now treat this 2x2 matrix as a 1x2x2 array instead, so that I can't do certain things (like multiply by another 2x2 matrix).

How do I get B as a 2x2 subarray of A?

Solution

  • If you think about a skyscraper, your A(1,:,:) is taking the first floor out and this operation inevitably happens across the 3rd dimension.

    You can use reshape(), squeeze() or permute() to get rid of the singleton dimension:

    reshape(A(1,:,:),2,2)
squeeze(A(1,:,:))
permute(A(1,:,:),[2,3,1])

    squeeze() pretty much does all the job by itself, however it is not an inbuilt function and in fact uses reshape(). The other two alternatives are expected to be faster.