Here is one way:
df <- data.frame(Value = rnorm(30),
Group = sample(c('A','B','C'), 30,
replace = TRUE))
## make a little function to do the job
iNumber <- function(f) {
f <- as.factor(f)
X <- outer(f, levels(f), "==")+0
rowSums(X * apply(X, 2, cumsum))
}
## add the numbering column
df <- within(df, internalNumber <- iNumber(Group))
## Check that it works
> head(df)
Value Group internalNumber
1 -1.5014788 C 1
2 0.6035679 C 2
3 -0.6953930 C 3
4 -0.2413863 A 1
5 -0.1170961 A 2
6 1.5110721 C 4
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf Of Robert Baer
Sent: Tuesday, 10 May 2011 7:22 AM
To: R-help@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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