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

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tban
tban on 9 Apr 2017
Commented: Walter Roberson on 10 Apr 2017
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.

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per isakson
per isakson on 9 Apr 2017
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Answers (3)


Star Strider
Star Strider on 9 Apr 2017
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

David Goodmanson
David Goodmanson on 9 Apr 2017
Edited: David Goodmanson on 9 Apr 2017
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.

  1 Comment

Walter Roberson
Walter Roberson on 10 Apr 2017
Element-wise multiplication:
matrix2 = matrix2 .* (matrix1 ~= 0);

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