Two other options (which I prefer):
aggregate(x~id.var, data=dat, FUN=min)

or
library(doBy)
summaryBy(x~id.var, data=dat, FUN=min)

HTH,
Ivan

Le 5/17/2011 16:09, John Kane a écrit :
A bit convolulted but it works

library(reshape2)
yy<- melt(dat)
zz<- cast(yy, id.var ~ variable, min)
zz[,2]

--- On Tue, 5/17/11, Downey, Patrick<pdow...@urban.org>  wrote:

From: Downey, Patrick<pdow...@urban.org>
Subject: [R] Minimum value by ID
To: r-help@r-project.org
Received: Tuesday, May 17, 2011, 9:44 AM
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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--
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Dept. Mammalogy
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calan...@uni-hamburg.de

**********
http://www.for771.uni-bonn.de
http://webapp5.rrz.uni-hamburg.de/mammals/eng/1525_8_1.php

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