selected<-sample(nrow(data),19811)
randomsample1 <- data[selected,]
randomsample2 <- data[-selected,] # for non select

But I think is good to have a variable that indicate selected and not selected cases in same data frame.

You can try this

selected<-rep(0,39622)
selected[sample(1:39622,39622/2)]<-1
data$selected<-selected
rm(selected)

or 

data$selected<-rbinom(39622,1,.5)


select case have the value 1, non-selected have value 0. In the second case, you will not get exactly .5 !
Justin BEM
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Tél (237) 99597295
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________________________________
De : Christopher David Desjardins <cddesjardins@gmail.com>
À : r-help@r-project.org
Envoyé le : Jeudi, 19 Mars 2009, 19h08mn 48s
Objet : [R] Randomly splitting a data frame in half

I have a data frame in long format and I would like to randomly divide this data frame in half. The data frame consists of 39622 rows and I initially tried ...

randomsample1 <- data[sample(nrow(data),19811), ]

Where allows me to randomly select half of the rows and assign them to randomsample1 but then I couldn't figure out how to select those rows that were not selected and assign them to randomsample2.

Please cc me if you reply as I'm a digest subscriber.
Thanks,
Chris

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