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How to use logical indexing for slices of a 3D matrix

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Anh Mai
Anh Mai on 21 Aug 2021
Commented: Anh Mai on 22 Aug 2021
Hi guys, I am trying to set value for different page of 3D matrix based on 2D matrix logical command.
domain=rand(4,4,4);
domain2D=domain(:,:,1);
condition1=domain2D>0.5;%logical index all elements of first slice that greater than 0.5
condition2=domain2D<0.2;%logical index all elements of first slice that smaller than 0.2
domain2D(condition)=10;%assign all elements that greater than 0.5 a value of 10.
domain2D(condition2)=5;%assign all elements that smaller than 0.2 a value of 5.
After running the above code, we get the following
condition1 =
4×4 logical array
1 0 1 0
1 0 0 0
1 1 1 1
0 0 0 0
>> condition2
condition2 =
4×4 logical array
0 1 0 1
0 0 0 0
0 0 0 0
1 0 0 0
As you can see, we can easily apply condition1 and condition2 for a 2D array, which is 1st slice of the 3D array, ie domain(:,:,1). Now, based on the condition1 and condition2, I like to use these logical indexes for any slices of the 3D array that i want. In this example, each elements falls within condition 1 a value of 10 for slice 1 to 3, and each elements falls within condition 2 a value of 5 from slice 1 to 2 of the 3D array. You can visualize the results as below picture.
If we are working with 2D array, it would be very simple as: domain(condition1)=10 (assume domain is 2D). How can we use this logical function in 3D array for any slices that I would like to?. Please help. Thank you all.

Accepted Answer

Adam Danz
Adam Danz on 22 Aug 2021
Edited: Adam Danz on 22 Aug 2021
nxm logical index applied to all pages of a 3D array
domain = rand(4,4,4);
condition1 = domain(:,:,1).*ones(size(domain)) > 0.5;
condition2 = domain(:,:,1).*ones(size(domain)) < 0.2;
domain(condition1) = 10;
domain(condition2) = 5
domain =
domain(:,:,1) = 10.0000 0.3294 0.2245 10.0000 0.4671 10.0000 10.0000 5.0000 0.2289 5.0000 0.2538 5.0000 0.2184 0.4877 10.0000 0.3680 domain(:,:,2) = 10.0000 0.9963 0.4664 10.0000 0.0913 10.0000 10.0000 5.0000 0.3675 5.0000 0.5812 5.0000 0.2225 0.0429 10.0000 0.0490 domain(:,:,3) = 10.0000 0.3901 0.6374 10.0000 0.0326 10.0000 10.0000 5.0000 0.0600 5.0000 0.7046 5.0000 0.3263 0.3253 10.0000 0.5136 domain(:,:,4) = 10.0000 0.2854 0.0045 10.0000 0.5459 10.0000 10.0000 5.0000 0.7885 5.0000 0.8755 5.0000 0.2039 0.3931 10.0000 0.7801
nxm logical indexing applied to specific pages of a 3D array
% tf is a n*m logical matrix
% pg is a row vector of pages
% output: matrix of linear indices
get3DInd = @(tf,pg)find(tf) + numel(tf)*(pg-1);
domain = rand(4,4,4);
condition1 = domain(:,:,1) > 0.5;
% Apply to pages 1 and 3
domain(get3DInd(condition1,[1,3])) = 10
domain =
domain(:,:,1) = 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 0.2942 10.0000 0.2943 0.2755 10.0000 0.2355 domain(:,:,2) = 0.4523 0.8492 0.3084 0.7687 0.1052 0.4329 0.3370 0.3450 0.8711 0.0895 0.8002 0.4803 0.5805 0.1339 0.6097 0.7108 domain(:,:,3) = 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 10.0000 0.7881 10.0000 0.0664 0.9131 10.0000 0.7121 domain(:,:,4) = 0.0603 0.1033 0.1447 0.3383 0.3646 0.5129 0.3797 0.9220 0.7900 0.1761 0.6725 0.6818 0.0385 0.4884 0.5942 0.2325
  5 Comments
Anh Mai
Anh Mai on 22 Aug 2021
Well, you are exceptionally genius. Thank you very much for your time and thorough explanation. U just saved me from a big headache.

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