How to pull values out of a matrix using another matrix?

Matrix_One:

[0 1 1 0;
 1 1 0 0;
 0 0 0 1;
 1 0 0 0]

Matrix_Two:

[9 3 1 9;
 3 6 1 9;
 2 0 6 4;
 2 6 4 8]

What I want:

[0 3 1 0;
 3 6 0 0;
 0 0 0 4;
 2 0 0 0]

Basically eliminate every value from initial matrix 2 beside where the ones are in matrix 1. I tried putting one matrix within another and it doesn't work.

[per isakson]

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Answer 1

[Walter Roberson]

matrix2(matrix1==0) = 0;

[David Goodmanson]

even better!

Answer 2

[Star Strider]

Try this:

M1 = [0 1 1 0;
    1 1 0 0;
    0 0 0 1;
    1 0 0 0];

M2 = [9 3 1 9;
    3 6 1 9;
    2 0 6 4;
    2 6 4 8];

Wanted = M2 .* M1

Wanted =
     0     3     1     0
     3     6     0     0
     0     0     0     4
     2     0     0     0

[tban]

very creative

[Star Strider]

... and it gives the desired result!

Answer 3

[David Goodmanson]

Hello Tudor, Here is one way:

indx = find(matrix1==0)
matrix2(indx) = 0

This works because 'find' looks at the elements of matrix1 columnwise in a running index from (in this case) 1 to 16, and the second line reads them back in the same way.

In this specific case you could multiply the two matrices by each other elementwise, but that way is a lot less versatile. In general maybe you would want to set the mat2 elements corresponding to mat1 = a to value b and you can't do that with elementwise multiplication.

[Walter Roberson]

Element-wise multiplication:

matrix2 = matrix2 .* (matrix1 ~= 0);