Re: how to extract entire rows from a data frame based on a index

[David L Lorenz <lorenz@usgs.gov>]

Paul,
  The aggregate() function should
do what you want. Look at the aggregate.data.frame() documentation. The
data would need to be sorted by date or ID and date to work correctly.
Dave

"Schwarz,Paul" <PSchwarz@gcrinsight.com>
Sent by: s-news-owner@lists.biostat.wustl.edu

11/17/2005 01:11 AM

To	<s-news@lists.biostat.wustl.edu>
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Subject	[S] how to extract entire rows from a data frame based on a index variable?

Dear S-News readers,

I'm puzzling over how to solve the following problem. Let's say that I
have a data frame with possibly multiple observations (rows) per
sampling unit, in this case survey respondents. In addition to a
respondent ID variable, there is a date variable tracking the date the
information was collected. If I want to extract from this data frame
only the most recent observation (or the first, etc.) for each
respondent, how could I do this? I've been thinking about this problem
as a tapply()-like problem where the respondent ID would be the index
variable, but I haven't figured how to write a function to extract and
return entire rows. I would appreciate ideas or suggestions from fellow
list readers on how to solve this.

Thanks for your help.

-Paul
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