multiplication of certain values using logical array

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

Answer 1

[Amit Davidi]

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?...)

[Rachel]

thanks, I knew there was a simple way somehow!

Answer 2

[Rachel]

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

Answer 3

[Amit Davidi]

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.