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

indexingMATLABmatrix

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!

Best Answer

result = p.*index_matrix

matrix multiplication is the keyword hehe

Best regards.

Related Solutions

MATLAB: Multiprod implementation question for matrix multiplication optimization

If we reshape your data slightly, we can use MTIMESX ( Download )

    >> A = cat(3,[1 2 3; 4 5 6; 7 8 9],[ 10 11 12; 13 14 15; 16 17 18])
    A(:,:,1) =
         1     2     3
         4     5     6
         7     8     9
    A(:,:,2) =
        10    11    12
        13    14    15
        16    17    18
    >> B=reshape(B,3,1,[])
    B(:,:,1) =
         1
         0
         0
    B(:,:,2) =
         0
         0
         1
    >> mtimesx(A,B)
    ans(:,:,1) =
         1
         4
         7
    ans(:,:,2) =
        12
        15
        18

MATLAB: Doesn’t A*P produce the right answer

MATLAB is producing the correct answer. Your understanding of matrix multiplication is not correct. The operation that gives the result you were expecting is actually

A*P.'

Also, you have a typo in your post. I assume the P you meant is:

P = [ 0 0 0 1; 1 0 0 0; 0 0 1 0; 0 1 0 0 ];

Best Answer

Related Solutions

MATLAB: Multiprod implementation question for matrix multiplication optimization

MATLAB: Doesn’t A*P produce the right answer

Related Question