# Re: [R] aggregate?

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Sat 18 Jun 2005 - 13:18:38 EST

On 6/17/05, alex diaz <celebridades@megamail.pt> wrote:
> Dear all:
>
> Here is my problem:
>
> Example data:
> dat<-data.frame(x=rep(c("a","b","c","d"),2),y=c(10:17))
>
> If I wanted to aggregate each level of column dat\$x I
> could use:
> aggregate(dat\$y,list(x=dat\$x),sum)
>
> But I just want to aggregate two levels ("c" and "d")
> to obtain a new level "e"
> I am expecting something like:
>
> x y
> 1 a 10
> 2 b 11
> 3 e 25
> 4 a 14
> 5 b 15
> 6 e 33

In the example
- dat\$y[3:4] are summed and
- dat\$y[7:8] are summed
so we assume that what is being requested is that "d" is to be replaced by "c" and runs of any level are to be summed.

To do that:
- create xx such that a, b, c and d in dat\$x are replaced with   with 1, 2, 3 and 3 in xx.
- in the second statement calculate a running sum except if the   last observation was the same as the current observation then   the Last Observation is Carried Forward (locf) so that all entries   in a run have the same number. e.g. in this case locf is   c(1, 2, 3, 3, 4, 5, 6, 6)
- Finally the 'by' collapses dat using locf rbinds the   resulting rows together to create a data frame.

xx <- ifelse(dat\$x == "d", 3, dat\$x)
locf <- cumsum(c(TRUE, xx[-1] != xx[-length(xx)])) f <- function(x) data.frame(x=x[1,1], y=sum(x[,2])) dat2 <- do.call("rbind", by(dat, locf, f))

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