Conditional selection of rows

Many thanks to Gabor Grothendieck, Thomas Lumley and James Holtman for their 
useful answers on this thread.  The three solutions worked for the problem.  
Here is a sumary of their responses (modified for consistency on notations):

F Z <gerifalte28 <at> hotmail.com> writes:
: Hi,
:
: I have a data.frame with several variables and 50,000 observations.
: i.e.
: data[1:2,1:7]
:   Iteration Day Production.Type tsUSusc tsASusc tsULat tsALat
:          1   0         Generic   17965 8833053      0      0
:          1   1         Generic   17965 8833053      0      0
:          .
:          .
:          .
:          1 199         Generic   17237 8141028     26  23131
:          2 127         Generic   15828 7307583     92  63463
:
: I would like to extract only the observations (rows) for the last "day" 
for
: each "iteration" and store them in a data frame.

Gabor Grothendieck's solution:


subset<-do.call("rbind", by(data, data$Iteration, tail, 1))


James Holtman's solution:

subset<- by(data, data$Iteration, function(x)x[nrow(x),])
subset<-do.call('rbind',subset)

Thomas Lumley's solution:

data <- data[order(data$Iteration, data$Day, decreasing=TRUE),]

subset <- data[!duplicated(data$Iteration),]

If you are sure that the data are in order to begin with you could just 
reverse the entire data set (  data <- data[nrow(data):1,] ), but I'm always 
reluctant to assume this.

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