Use logical values to extract a matrix not a vector?

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   1   0
     1   1   0
     1   1   0
     0   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

K>> ext

ext =

       6     2
       7     2
       9     3

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.

[Guillaume]

There's no generic way as in most cases it would not be possible to get a matrix. E.g. what would the output be if your logical array was

     1   1   0
     0   0   1
     1   1   0
     0   0   0
     0   1   0

instead?

Why is reshape forbidden? What other functions are forbidden?

[Xiaohan Du]

then is it possible to have the corresponding value at logical 1 and 0 at logical 0, so the resulting matrix has the same size as original one?

[Xiaohan Du]

can it be done if reshape is not forbidden?

[Dennis]

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

ext=M.*logi

Answer 1

[Guillaume]

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))

Answer 2

[Star Strider]

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

Answer 3

[Fernando Feijoo]

M.*logi