Use logical values to extract a matrix not a vector?

Question

Xiaohan Du 2018-6-18

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回答： Fernando Feijoo 2019-11-4

Hi all,

I have a matrix like this:

M =
           6     2     9
           7     2     3
           9     3    10
          10     9     4
           6     3     2

and a logical array like this:

logi =
    5×3 logical array
 1   0
 1   0
 1   0
 0   0
 0   0

If I extract associated values from M using logi:

K>> ext = M(logi)
ext =
       6
       7
       9
       2
       2
       3

but what I want is actually

So is there a way to extract the values into a matrix using logical operators? Using reshape or whatever to reshape the vector into matrix is forbidden in my case.

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Xiaohan Du 2018-6-18

can it be done if reshape is not forbidden?

Dennis 2018-6-18

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If your 2 matrices have the same size in the first place:

ext=M.*logi

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Answer 1

Guillaume 2018-6-18

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This is going to break in all sort of interesting ways if your logical array wouldn't result in a square matrix. Detecting such situations is left as an exercise to the reader...

[r, c] = find(logi);
ext = M(unique(r), unique(c))

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Answer 2

Star Strider 2018-6-18

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This will work for this particular problem and perhaps for similar situations. It will not work generically:

A = M.*logi;
C = num2cell(A);
I = cellfun(@eq, C, num2cell(zeros(size(C))));
C(I) = {[]};
ext = cell2mat(C)
ext =
     6     2
     7     2
     9     3