Using a binary matrix as an index to select values from another matrix

Hi all,

I think this is a really beginner question, but I can't seem to find the relevant answers online.

If I have the following matrix:

% 3x2
index_matrix = [0,0;1,0;1,1;]

% 3x2
p = [13,12;11,19;22,23;]

I wish to select the corresponding value in p if the value at the same position in index_matrix = 1.

If the value in index_matrix is 0, then the corresponding value should be set to 0.

That is, I wish to have the following:

result = [0,0;11,0;22,23;]

Thank you in advance!

Answer 1

[Kai Franke]

result = p.*index_matrix

matrix multiplication is the keyword hehe

Best regards.

[Stephen23]

"matrix multiplication is the keyword hehe"

Actually the keyword is element-wise multiplication (that is what you used in your answer).

Matrix multiplcation is something quite different, as the MATLAB documentation explains:

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

[Shuyi Jia]

Thanks! That works!

Just a follow-up question, what if my index_matrix is like this:

index_matrix = [1,0;3.5,1;0,6;]

p = [13,12;11,19;22,23;]

And I still wish to obtain the corresponding values in p, only if the values at the same positions in index_matrix == 1 (not 3.5 or 6; 0 for all other values)?

That is:

result = [13,0;0,19;0,0;]

[Kai Franke]

@Stephen Cobeldick

oh yeah you are absolutely right! Element-wise multiplication. My answer was very sloppy, my bad. Thanks for the correction!

Answer 2

[Stephen23]

>> X = [1,0;3.5,1;0,6];
>> p = [13,12;11,19;22,23];
>> p.*(X==1)
ans =
   13    0
    0   19
    0    0

[Shuyi Jia]

Thank you! I didn't know one can manipulate matrix like this. New to matlab!