see package plyr, especially the function ddply(), eg.., in your case:

ddply(dataframe, .(columnA, columnB), summarise, 
columnC = length(columnB)
)

Scott
On Wednesday, March 2, 2011 at 9:10 AM, Simone Gabbriellini wrote: 
> Hello List,
> 
> I would like to simplify a data.frame like this
> 
> columnA columnB
> user10 proj12
> user10 proj19
> user10 proj12
> 
> into something like:
> 
> columnA columnB columnC
> user10 proj12 2
> user10 proj19 1
> 
> I know unique() can simplify the data.frame, but how to count and store the duplicates?
> 
> thanks in advance for any help.
> 
> best regards,
> Simone
> 
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