[R] problem with sum function

[Mark A. Albins]

Hi!

I'm running R version 2.13.0 (2011-04-13)
Platform: i386-pc-mingw32/i386 (32-bit)

When i type in the command:

sum(c(-0.2, 0.8, 0.8, -3.2, 1.8))

R returns the value:

-5.551115e-17

Why doesn't R return zero in this case?  There shouldn't be any rounding 
error in a simple sum.

Thanks,

Mark