[R] Creating Observation ID

[Bill.Venables at csiro.au]

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-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Robert Baer
Sent: Tuesday, 10 May 2011 7:22 AM
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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