hadley wickham <h.wickham <at> gmail.com> writes:
>
> You might also want to take a look at the plyr package:
>
> install.packages("plyr")
> library(plyr)
> ddply(x, .(A, D), function(df) sum(df$Z))
> dlply(x, .(A, D), function(df) sum(df$Z))
>
I recently for the first time used the plyr package in an introductory
course as a general replacement of for "by", "(x)apply", followed by
do.call gymnastics. It was an amazing step forward to have a nicely
orthogonal set of function that returns what people expected instead
of converting everything to a list.
I know, real MEN program assembler and love (x)apply...
Dieter
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