Oops,
I am terribly sorry, I recognized my mistake, as I thought
that the factors are ordered in PPress,
1 1 (k times)
1 2 (k times)
1 3 (k times)
etc...
Of course, "solution" I supplied is then not correct.
Thank you for your notice and for a more general solution.
Jan
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/- Jan Svatos, Sokolovska 855/225 -/
/- Data Analyst, Prague 9 -/
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/- jan_svatos at eurotel.cz Czechia -/
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"Dieter Menne"
<dieter.menne at menne- <Jan_Svatos at
eurotel.cz>
biomed.de>
07/31/01 11:36 AM AW: [R] Applying
"by" results
Jan,
I tried again, and I am sorry to say that your result is not correct.
I would only work if the data are ordered from the beginning
in the same order as they are processed.
See below: Press1 is not correct (not easy to see).
Press 2 is correct.
set.seed(13)
PPress<-data.frame(Marker=as.factor(c(rep(1,12),rep(2,12))),
Chan=as.factor(rep(1:4,6)), Press=rnorm(24) )
attach(PPress)
PPress$Press1<-unlist(by (Press,list(Marker,Chan), function(x) x-min(x)))
minp<-unlist(tapply(Press,list(Chan,Marker),min,simplify=F))
imin<-tapply(Press,list(Chan,Marker))
PPress$Press2<-Press-minp[imin]
Dieter
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