have a look at help file of function tapply(), and try this:
with(dat, tapply(x, id.var, min))
I hope it helps.
Best,
Dimitris
On 5/17/2011 3:44 PM, Downey, Patrick wrote:
Hello,
I have a longitudinal dataset where each individual has a different number
of entries. Thus, it is of the following structure:
x<- runif(12)
id.var<- factor(c(rep("D1",4),rep("D2",2),rep("D3",3),rep("D4",3)))
dat<- as.data.frame(x)
dat$id.var<- id.var
dat
dat
x id.var
1 0.9611269 D1
2 0.6738606 D1
3 0.9724301 D1
4 0.9787778 D1
5 0.2468355 D2
6 0.7031734 D2
7 0.2458727 D3
8 0.8439799 D3
9 0.5223196 D3
10 0.6930475 D4
11 0.8887677 D4
12 0.5483756 D4
I want to create a vector with length equal to the number of unique id.var
and which has the minimum value for each id.var. That is, I want a vector
which holds the minimum value for each person in my dataset.
The following works, but I'm sure there is something more efficient. I
would assume there is a function for this, but couldn't find anything.
id<- levels(id.var)
min<- rep(0,length(id))
for(i in 1:length(id)){
min[i]<- min(dat$x[dat$id.var==id[i]])
}
min
min
[1] 0.6738606 0.2468355 0.2458727 0.5483756
Thank you in advance,
Mitch
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--
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center
Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014
Web: http://www.erasmusmc.nl/biostatistiek/
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
