[R] aggregation question

[Sundar Dorai-Raj]

Christoph Lehmann wrote on 4/15/2005 9:51 AM:
> Hi I have a question concerning aggregation
> 
> (simple demo code S. below)
> 
> I have the data.frame
> 
>    id        meas date
> 1   a 0.637513747    1
> 2   a 0.187710063    2
> 3   a 0.247098459    2
> 4   a 0.306447690    3
> 5   b 0.407573577    2
> 6   b 0.783255085    2
> 7   b 0.344265082    3
> 8   b 0.103893068    3
> 9   c 0.738649586    1
> 10  c 0.614154037    2
> 11  c 0.949924371    3
> 12  c 0.008187858    4
> 
> When I want for each id the sum of its meas I do:
> 
>     aggregate(data$meas, list(id = data$id), sum)
> 
> If I want to know the number of meas(ures) for each id I do, eg
> 
>     aggregate(data$meas, list(id = data$id), length)
> 
> NOW: Is there a way to compute the number of meas(ures) for each id with 
> not identical date (e.g using diff()?
> so that I get eg:
> 
>   id x
> 1  a 3
> 2  b 2
> 3  c 4
> 
> 
> I am sure it must be possible
> 
> thanks for any (even short) hint
> 
> cheers
> Christoph
> 
> 
> 
> --------------
> data <- data.frame(c(rep("a", 4), rep("b", 4), rep("c", 4)),
>                    runif(12), c(1, 2, 2, 3, 2, 2, 3, 3, 1, 2, 3, 4))
> names(data) <- c("id", "meas", "date")
> 
> m <- aggregate(data$meas, list(id = data$id), sum)
> names(m) <- c("id", "cum.meas")
> 


How about:

m <- aggregate(data["date"], data["id"],
                function(x) length(unique(x)))

--sundar