[R] counting numbers without replicates in a vector

[Ray Brownrigg]

> I am just wondering if there is an easy way to count
> in a numeric vector how many numbers don't have
> replicates. 
> For example, 
> a=c(1,1,2,2,3,4,5), how can I know there are three
> numbers (3, 4 and 5) without replicates?
> 
How about:
length(table(a)[table(a) == 1])

?
Ray Brownrigg