Change array elements that meet a condition - Multidimensional array case

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RD 2022-1-21

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https://ww2.mathworks.cn/matlabcentral/answers/1633595-change-array-elements-that-meet-a-condition-multidimensional-array-case

评论： Rik 2022-1-21

I can handle this in the case of two-dimensional arrays.

I have trouble in the case of multidimensional arrays. For example

A=ones(4,4,2);
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

A=reshape(A,16,2);
B= reshape(B,16,1);
A(B==3,:)=0;
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.

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Answer 1

Rik 2022-1-21

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I can think of two ways:

using a loop
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.

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RD 2022-1-21

编辑：RD 2022-1-21

So maybe my solution with reshape is the optimum because no for loop is required and doesn't affect physical memory. Am I right?

Rik 2022-1-21

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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