Change array elements that meet a condition - Multidimensional array case

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RD on 21 Jan 2022
Commented: Rik on 21 Jan 2022
I can handle this in the case of two-dimensional arrays.
I have trouble in the case of multidimensional arrays. For example
B=zeros(4); B(2:4,2:4)=3;
I would like to change the value of all elements on each page of A this way
B= reshape(B,16,1);
A=reshape(A, 4, 4, 2)
A =
A(:,:,1) = 1 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 A(:,:,2) = 1 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0
Is it possible to obtain that result without reshapes?
In general, how to handle conditions that apply only to some indexes in multidimensional arrays?
PS: A and B are very large data matrices.

Answers (1)

Rik on 21 Jan 2022
I can think of two ways:
  1. using a loop
  2. replicating B in the third dimension to match the size of A
I suspect the first way would be faster, but I think that might depend on the sizes of the array and the physical memory.
Rik on 21 Jan 2022
A for loop may actually be very efficient, unless the third dimension is large relative to the product of the other two. I don't think the reshape calls are that cheap in the end.
The only way to know for sure is to test it.

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