Converting positives to negatives in a column.

I have somewhat of a confusing problem that I need help with. I am going to explain it the best that I can, so here goes. I have two column vectors, one is Cp the others is Cf. The Cp column has positive values and some negative values. I basically need to take the rows in the Cp column that are negative and make the same rows of Cf negative. For example:

 Cp=[1;2;-3;-4;5]

 Cf=[1;3;5;7;9]

Then the new column vector would be:

 new_vector=[1;3-5;-7;9]

Basically making any negative row in Cp, negative in Cf.

Is there any easy way to do this? Any help is much appreciated.

Answer 1

[Walter Roberson]

Cf(Cp<0) = -Cf(Cp<0);