[R] obtaining first and last record for rows with same identifier

[Francisco J. Zagmutt]

If you want to obtain a data frame you can use the functions head and tail 
like:

dat=data.frame(id=rep(1:5,3),num=rnorm(15), num2=rnorm(15))#Creates data 
frame with id
last=do.call("rbind",by(dat,dat$id,tail,1))#Selects the last observation for 
each id
first=do.call("rbind",by(dat,dat$id,head,1))#Selects the first observation 
for each id
newdat=rbind(first,last)#Joins data
newdat=newdat[order(newdat$id),]#sorts data by id

Notice that rownames will give you the original row location of the 
observations selected

I hope this helps

Francisco


>From: Berton Gunter <gunter.berton at gene.com>
>To: "'Sean Davis'" <sdavis2 at mail.nih.gov>, <sms13+ at pitt.edu>
>CC: "'rhelp'" <r-help at stat.math.ethz.ch>
>Subject: RE: [R] obtaining first and last record for rows with same 
>identifier
>Date: Tue, 24 May 2005 12:17:58 -0700
>
>
>I think by() is simpler:
>
>  by(yourframe,factor(yourframe$patid),function(x)x[c(1,nrow(x)),])
>
>
>
>-- Bert Gunter
>Genentech Non-Clinical Statistics
>South San Francisco, CA
>
>"The business of the statistician is to catalyze the scientific learning
>process."  - George E. P. Box
>
>
>
> > -----Original Message-----
> > From: r-help-bounces at stat.math.ethz.ch
> > [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Sean Davis
> > Sent: Tuesday, May 24, 2005 11:38 AM
> > To: sms13+ at pitt.edu
> > Cc: rhelp
> > Subject: Re: [R] obtaining first and last record for rows
> > with same identifier
> >
> > If you have your data.frame ordered by the patid, you can use the
> > function rle in combination with cumsum.  As a vector example:
> >
> >  > a <- rep(c('a','b','c'),10)
> >  > a
> >   [1] "a" "b" "c" "a" "b" "c" "a" "b" "c" "a" "b" "c" "a" "b" "c" "a"
> > "b" "c" "a"
> > [20] "b" "c" "a" "b" "c" "a" "b" "c" "a" "b" "c"
> >  > b <- a[order(a)]
> >  > b
> >   [1] "a" "a" "a" "a" "a" "a" "a" "a" "a" "a" "b" "b" "b" "b" "b" "b"
> > "b" "b" "b"
> > [20] "b" "c" "c" "c" "c" "c" "c" "c" "c" "c" "c"
> >  > l <- rle(b)$length
> >  > cbind(l,cumsum(l),cumsum(l)-l+1)
> >        l
> > [1,] 10 10  1
> > [2,] 10 20 11
> > [3,] 10 30 21
> >
> > # use the line below to get the length of the block of the dataframe,
> > the start, and then end indices
> >  > cbind(l,cumsum(l)-l+1,cumsum(l))
> >        l
> > [1,] 10  1 10
> > [2,] 10 11 20
> > [3,] 10 21 30
> >  >
> >
> > Sean
> >
> >
> > On May 24, 2005, at 2:27 PM, sms13+ at pitt.edu wrote:
> >
> > > I have a dataframe that contains fields such as patid, labdate,
> > > labvalue.
> > > The same patid may show up in multiple rows because of lab
> > > measurements on multiple days.  Is there a simple way to
> > obtain just
> > > the first and last record for each patient, or do I need to
> > write some
> > > code that performs that.
> > >
> > > Thanks,
> > > Steven
> > >
> > > ______________________________________________
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