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Reshaping a 4D array to 2D

From: Brett Green
Subject: Reshaping a 4D array to 2D
Date: Thu, 12 Mar 2020 15:42:20 -0400

Hello all,

I need to reshape a 4D array H indexed as H(x,y,n,data) to a 2D array D indexed as D(u,data) where u consolidates the indices x, y and n. I can do this with a simple demonstration matrix as follows:

for x=1:2
for y=1:3
for n=1:4
for data=""> H(x,y,n,data) = x*1000+y*100+n*10+data;
D = reshape(H,24,5)

It is critical that the last index, data, is not rearranged in any way - H(x,y,n,1) and H(x,y,n,2) must be reshaped to D(u,1) and D(u,2) so that (1) they are indexed by the same number for data, and (2) there is a consistent mapping x,y,n -> u, so that u depends only on x,y,n and not on data.

reshape appears to satisfy both of these - the columns of D share the same value for data, and each row of D has the same values for x,y,n for all entries. The documentation simply states that column-major order is used, which if I understand correctly preserves the last index (in the case of a matrix the column, in this case data), as I wanted.

I have never worked with reshaping arrays before, so I will continue to try to understand why this works. In the meantime, would someone who understands better be able to confirm for me that reshape does indeed satisfy both the requirements I mentioned above?

- Brett Green

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