[R] Memory usage of R on Windows XP

[Peter Wilkinson]

I am running R 1.8.1 on windows xp. I have been using the 'apply' R 
function to run a short function against a matrix of 19000 x 340 rows ... 
yes it is a big matrix. every item in the matrix is a float that can have a 
maximum value of 2^16 ~ 65k.

The function:

mask256 <- function(value) {
     if (value < 256) {
        result = 0
     }
     else {
        result = 1
     }
     result
}

what happens is that the memory required for the session to run starts 
ballooning. The matrix with a few other objects starts at about 160M, but 
then quickly goes up to 750M, and stays there when the function has completed

I am fairly new to R. Is there something I should know about writing 
functions , i.e. do I need to clean-up at the end of the function? It seems 
R can not release the memory once it has been used. When I close the R 
application and open the R application again then the memory is back down 
to what it is supposed to be, the size of the workspace, plus any new 
objects that I have created

Does anybody know what is going on?

Peter