[R] how to create data.frames from vectors with duplicates

[zhenjiang xu]

Hi R users,

suppose I have two vectors,
 > x=c(1,2,3,4,5)
 > y=c('a','b','c','a','c')
How can I get a data.frame like this?
> xy
      count
a     5
b     2
c     8

I know a few ways to fulfill the task. However, I have a huge number
of this kind calculations, so I'd like an efficient solution. Thanks

-- 
Best,
Zhenjiang