[R] long format - find age when another variable is first 'high'

[ONKELINX, Thierry]

Dear David,

You would speed up things is you first create a subset were all values
of ldlc is >= 130. Then you only have to find the lowest age for each
child in this subset.

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens David Freedman
Verzonden: maandag 25 mei 2009 14:45
Aan: r-help at r-project.org
Onderwerp: [R] long format - find age when another variable is first
'high'


Dear R, 

I've got a data frame with children examined multiple times and at
various ages.  I'm trying to find the first age at which another
variable
(LDL-Cholesterol) is >= 130 mg/dL; for some children, this may never
happen. 
I can do this with transformBy and ddply, but with 10,000 different
children, these functions take some time on my PCs - is there a faster
way to do this in R?  My code on a small dataset follows.  

Thanks very much, David Freedman

d<-data.frame(id=c(rep(1,3),rep(2,2),3),age=c(5,10,15,4,7,12),ldlc=c(132
,120,125,105,142,160))
d$high.ldlc<-ifelse(d$ldlc>=130,1,0)
d
library(plyr)
d2<-ddply(d,~id,transform,plyr.minage=min(age[high.ldlc==1]));
library(doBy)
d2<-transformBy(~id,da=d2,doby.minage=min(age[high.ldlc==1]));
d2
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