How to include custom na.action in function

Hello,

I?ve tried googling for an answer to this but I simply can?t find something that fixes my problem. I have a long numerical vector with positive, negative and null values. I want to revert the sign of the positive and negative values and for zero to remain zero. I?ve written a function that works, except that my vector has missing values (NA) and I need to keep those as missing values. How could I add that to this function:

revertsign<-function(x){
 if (x > 0) {x <- x*-1}
   else 
if (x < 0) {x <- abs(x)}
 else
   if (x == 0) {x <- 0}
 }

I?ve tried if(is.na(x)) {x <- NA} but I get the following error message: Error in if (x > 0) { : missing value where TRUE/FALSE needed. Which I guess is the first NA in the vector which fails the first if of the function.

I use supply() to run the function on a vector.

Thanks for any assistance.

Cheers

Alejandro