Creating Observation ID

Does the following work for you?

df2 <- transform(df, ObsID=ave(rep(0,length(Group)), Group,

FUN=seq_along))

head(df2)

       Value Group ObsID
1 -0.0025132     B     1
2 -1.2456156     A     1
3 -2.0531704     B     2
4  1.5861770     B     3
5  0.1900908     A     2
6  0.7197067     B     4

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-help-bounces at r-project.org
[mailto:r-help-bounces at r-project.org] On Behalf Of Robert Baer
Sent: Monday, May 09, 2011 2:22 PM
To: R-help at r-project.org
Subject: [R] Creating Observation ID

If I have a data frame something like:
Value=rnorm(30)
Group = sample(c('A','B','C'), 30, replace=TRUE)
df = data.frame(Value, Group)

It seems like it should be simple to create an 'ObsID' column
which indicates the observation order of each Value within
each of the 3 groups.  Somehow, I can't quite see how to do
it without manually sub-setting the parent data frame and
then putting it back together again.

Anyone able to get me started on a cleaner (more R-like) approach?

Thanks,

Rob

------------------------------------------
Robert W. Baer, Ph.D.
Professor of Physiology
Kirksville College of Osteopathic Medicine
A. T. Still University of Health Sciences
800 W. Jefferson St.
Kirksville, MO 63501
660-626-2322
FAX 660-626-2965

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