One way is to use the following functions:
?unique
?aggregate
?merge

For example,
> x.df <- data.frame(Group=LETTERS[c(1, 1, 1, 2, 3)], Value=c(1, 1, 2, 2,
0))
> x.agg <- aggregate(x$Value, list(Group=x$Group),
function(x){length(unique(x))})
> x.agg
  Group x
1     A 2
2     B 1
3     C 1
> x.merged <- merge(x.df, x.agg, by="Group")
> x.merged
  Group Value x
1     A     1 2
2     A     1 2
3     A     2 2
4     B     2 1
5     C     0 1
>


On Dec 9, 2007 1:35 AM, christopher snow <snowch@coralms.com> wrote:

> I have a table with two columns:
>
> A   1
> A   1
> A   2
> B   2
> C   0
>
> I would like to produce a third column that contains the counts of each
> unique combination of col1 and col2:
>
> A   1  2
> A   1  2
> A   2  1
> B   2  1
> C   0  1
>
> How can I do this in R?
>
> Thanks in advance ...
>
>
> --
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>



-- 
======================================
T.K. (Tae-kyun) Kim
Ph.D. student
Department of Marketing
Marshall School of Business
University of Southern California
======================================

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