[R] Calculating SD according to groups of rows

[Petr PIKAL]

What about aggregate.

with(dat, aggregate(HR, list(sub_id=SUBJECT_ID), sd))

shall result in required final output form.

Regards

Petr

r-help-bounces at r-project.org napsal dne 20.11.2008 09:20:36:

> pufftissue pufftissue <pufftissue <at> gmail.com> writes:
> 
> > 
> > What I am getting is indeed:
> > 
> > 7200          23955        34563        8934
> > 16.39977 10.03896    11.234      14.02
> > 
> > I'd like the final output to be:
> > 
> > subject_id         hr_Stand_Deviation
> > 7200                  16.39977
> > 23955                10.03896
> > 34563                11.234
> > 8934                  14.02
> > 
> 
> The hard way could go like that; I personally got used to it, but I 
admit
> it is one of the thinks that are unusually difficult in R.
> 
> dat = data.frame(SUBJECT_ID=sample(letters[1:5],100,TRUE),HR=rnorm(100))
> sd.list = with(dat, tapply(HR, SUBJECT_ID, sd))
> data.frame(SUBJECT_ID=rownames(sd.list),sd=sd.list)
> 
> I think Hadley Wickham tried to make life easier with the plyr package,
> so I thought something like the below would work out of the box.
> However, there must be something wrong with the syntax, the 
> result is only "approximately" correct.
> 
> Dieter
> 
> library(plyr)
> daply(dat,.(SUBJECT_ID),sd)
> ddply(dat,.(SUBJECT_ID),sd)
> 
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