[R] Quantiles of data in a contingency table

[Matt Mohebbi]

Hello, 

I have data of the following form:

> data <- data.frame(type=c("c","d","e"), size=c(10,20,30), count=c(20,10,5))
> data
  type size count
1    c   10    20
2    d   20    10
3    e   30     5

I would like to compute the quantiles of size given the counts. For
instance, in this example, the median size would be 10. Is there an
easy way of doing this?

Is there a good way to deal with data in this format in general? Much
of R seems to center around having an entry for each item. This
question (http://www.r-project.org/nocvs/mail/r-help/2000/0102.html)
seems to be related but no one provided an answer.

Thanks, 
Matt