Column indexing with condition

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Duc Anh Le
Duc Anh Le 2020-4-27
回答： per isakson 2020-4-28
I have two matrices
A = [1, 0, 0;
0, -2, 0;
0, 0, -3];
B =[1, 2, 3;
4, 5, 6;
7, 8, 9];
I need to find columns in A with negative values (in this example it's 2 and 3 columns) and change the values to opposite in the corresponding columns of B. Basically, I want
B = [1, -2, -3;
4, -5, -6;
7, -8, -9];
Is there a fast way to do this using indexing? I know how to do this with a for loop, but I believe that's not the best solution

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per isakson
per isakson 2020-4-28
Try this
>> isn = any( A<0, 1 );
>> B(:,isn) = -B(:,isn)
B =
1 -2 -3
4 -5 -6
7 -8 -9
and read about Indexing with Logical Values (in the middle of the page)

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