You could try using merge:
>
d<-data.frame(Subject=rep(11:13,each=3),Time=101:109,Marker=c(0,1,0,
0,0,0, 0,0,1))
> d
Subject Time Marker
1 11 101 0
2 11 102 1
3 11 103 0
4 12 104 0
5 12 105 0
6 12 106 0
7 13 107 0
8 13 108 0
9 13 109 1
> d$Time - merge(d,d[d$Marker==1,],by="Subject",all.x=TRUE)$Time.y
[1] -1 0 1 NA NA NA -2 -1 0
If you want the reference times for Subjects without a
marked instance to be 0, replace the NA's in Time.y by 0:
> NAtoZero<-function(x){ x[is.na(x)]<-0 ; x }
> d$Time -
NAtoZero(merge(d,d[d$Marker==1,],by="Subject",all.x=TRUE)$Time.y)
[1] -1 0 1 104 105 106 -2 -1 0
If there is more than one marked instance for
a given subject this method will fail.
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
> -----Original Message-----
> From: r-help-boun...@r-project.org
> [mailto:r-help-boun...@r-project.org] On Behalf Of Dennis Fisher
> Sent: Friday, December 04, 2009 2:47 PM
> To: r-h...@stat.math.ethz.ch
> Subject: [R] Use of apply rather than a loop
>
> Colleagues,
>
> R 2.9.0 on all platforms
>
> I have a dataset that contains three columns of interest:
> ID's, serial
> elapsed times, and a marker. Representative data:
> Subject Time Marker
> 1 100.5 0
> 1 101 0
> 1 102 1
> 1 103 0
> 1 105 0
>
> For each subject, I would like to find the time associated
> with MARKER
> == 1, then replace Time with Time - (Time[Marker == 1])
> The result for this subject would be:
> Subject Time Marker
> 1 -1.5 0
> 1 -1 0
> 1 0 1
> 1 1 0
> 1 3 0
>
> One proviso: some subjects do not have Marker == 1; for these
> subjects, I would like Time to remain unchanged.
>
> At present, I am looping over each subject. The number of
> subjects is
> large so this process is quite slow. I assume that one of the apply
> functions could speed this markedly but I am not facile with them.
> Any help would be appreciated.
>
> Dennis
>
> Dennis Fisher MD
> P < (The "P Less Than" Company)
> Phone: 1-866-PLessThan (1-866-753-7784)
> Fax: 1-866-PLessThan (1-866-753-7784)
> www.PLessThan.com
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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and provide commented, minimal, self-contained, reproducible code.
