[R] Find last row (observation) for each combination of variables

[Berton Gunter]

Leif:

Rather than trying to mimic what you might do in SAS take advantage of R's
ability to use arbitrary data structures, e.g. lists. So, one approach is:

(your.df is the data frame)

your.list<-split(your.df,your.ts[,1:3],drop=TRUE)
t(sapply(your.list,function(x)x[which.max(x$TS),]))

Cheers,
Bert

-- 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 Leif 
> Kirschenbaum
> Sent: Tuesday, January 10, 2006 1:04 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Find last row (observation) for each combination 
> of variables
> 
> Let's say I have a data.frame like
> A	B	C	TS	other columns
> 1	1	1	12345
> 1	1	1	56789
> 1	2	1	23456
> 1	2	2	23457
> 2	4	7	23458
> 2	4	7	34567
> 2	4	7	45678
> 
> and I want the last row for each unique combination of A/B/C, 
> where by "last" I mean greatest TS.
> A	B	C	TS	other columns
> 1	1	1	56789
> 1	2	1	23456
> 1	2	2	23457
> 2	4	7	45678
> 
> I did this simply in SAS:
>  proc sort data=DF;
>    by A B C descending TS
>  run;
>  proc sort data=DF NODUPKEY;
>    by A B C;
>  run;
> 
> I tried using "aggregate" to find the maximum TS for each 
> combination of A/B/C, but it's slow.
> I also tried "by" but it's also slow.
> My current (faster) solution is:
> 
>  DF$abc<-paste(DF$A,DF$B,DF$C,sep="")
>  abclist<-unique(DF$ABC)
>  numtest<-length(abclist)
>  maxTS<-rep(0,numtest)
>  for(i in 1:numtest){
>   maxTS[i]<-max(DF$TS[DF$abc==abclist[i]],na.rm=TRUE)
>  }
>  maxTSdf<-data.frame(device=I(abc),maxTS=maxTS )
>  DF<-merge(DF,maxTSdf,by="abc",all.x=TRUE)
>  DF<-Df[DF$TS==DF$maxTS,,drop=TRUE]
>  DF$maxTS<-NULL
> 
> This seems a bit lengthy for such a simple task.
> 
> Any simpler suggestions?
> 
> -Leif K.
> 
> Leif Kirschenbaum
> Senior Yield Engineer
> Reflectivity, Inc.
> (408) 737-8100 x307
> leif at reflectivity.com
> 
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