R help - Mar 2010 - How can I return rows from a data frame with maximum value by factor?

Hi,

I'm new to R and new to this forum. I'm struggling with trying to
extract
certain rows of data from my data.frame. The data.frame has eleven columns.
Among those columns are "FISH_ID" and "DATE_TIME". FISH_ID
is a factor. For
each of my 21 unique FISH_IDs (levels) I have a few to a few thousand rows,
each row with a unique DATE_TIME value. I would like to obtain, for each
FISH_ID, the entire record from the data.frame with the latest DATE_TIME
value (i.e. max (DATE_TIME). Thus, the resulting subset would be the
original eleven columns by 21 rows (one for each level of my FISH_ID). 

Any advice would be greatly appreciated.

Nick
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Solved my own question by looking at some other threads. For those who may be
intersted, here was the solution to my problem.

do.call(rbind,lapply(split(data.frame,FISH_ID),function(x)x[which.max(x$DATE_TIME),]))
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For future reference you may want to look at the plyr package, or even the
sapply and aggregate functions and possibly simpler ways to do what you did
below.

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Nick Ackerman
> Sent: Wednesday, March 17, 2010 11:45 AM
> To: r-help at r-project.org
> Subject: Re: [R] How can I return rows from a data frame with maximum
> value by factor?
> 
> 
> Solved my own question by looking at some other threads. For those who
> may be
> intersted, here was the solution to my problem.
> 
> do.call(rbind,lapply(split(data.frame,FISH_ID),function(x)x[which.max(x
> $DATE_TIME),]))
> --
> View this message in context: http://n4.nabble.com/How-can-I-return-
> rows-from-a-data-frame-with-maximum-value-by-factor-
> tp1596777p1596853.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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