multiplication of certain values using logical array

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Rachel
Rachel 2012-2-2
编辑： Jon 2013-9-26
Hi all,
I have a matrix from which I have derived a logical array. What I'd like to do is multiply true values in the original matrix by -1. I can change these values to a constant value but I can't figure out how to multiply the existing value.
I should add that I'm after a vectorisation rather than loops to quicken my script.
thanks in advance,
Rachel

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采纳的回答

Amit Davidi
Amit Davidi 2012-2-2
If A is your original matrix and fltr is the derived logical array (having the same dimensions as A), than:
A(fltr) = -A(fltr);
Should do the trick.
(I think Andrei might have misunderstood your question. Or did I?...)

更多回答（2 个）

Rachel
Rachel 2012-2-2
but this changes every value? (andrei) - I need to change just the values which are true in the logical.e.g.
A = 1,2,3;4,5,6;7,8,9 % original matrix
logical = 1,0,0;0,1,1;1,1,0 % logical array
B = -1,2,3;4,-5,-6;-7,-8,9 % A where the values sharing the same position of logical true have been multiplied by -1.
thanks
Rachel
Amit Davidi
Amit Davidi 2012-2-2
Of course, it'll change only the values where the logical array is true. Note, however, that the command
B = -A(fltr);
would not give you what you want! This will result in a column vector of the changed values.

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