[R] Conditioned file Import?

Robert Schick Robert.Schick at noaa.gov
Mon Jan 27 20:38:02 CET 2003

I'm using R 1.6.2 on Windows 2000.

I have two similar sets of files in two different directories. One
contains species presence data and environmental measurements; the other
contains species absence data and environmental measurements for the
same variables as the presence data. 

The absence datasets contain many more observations than the presence
datasets, and I'd like to draw a random sample from these larger
datasets upon import of the text files. I would also like to condition
the import so that I only draw rows whose column(s) satisfy a criteria,
e.g. where data$temp <= 0. However, I'm not seeing anyway of doing this. 

I know I can use something like:

> test <- read.table(file="10.06.94ed.txt",header=T)
> test2 <- test[test$temp>15,]
> test2[sort(sample(10)),]

but can I do this as I read in the file? If yes, how could I find the #
of lines in my smaller file, and then pass that number to the sample
call on the larger file? (Is there a wc -l equivalent in R?)

Finally, if yes to either of the above, is there a way to batch this?
Rob Schick
NOAA Fisheries
Santa Cruz Lab
110 Shaffer Road
Santa Cruz, CA 95060
Phone: 831.420.3960

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