Column indexing with condition

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

Answer 1

[per isakson]

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)