R help - Jan 2003 - Conditioned file Import?

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
Ecologist
NOAA Fisheries
Santa Cruz Lab
110 Shaffer Road
Santa Cruz, CA 95060
Phone: 831.420.3960

On Mon, 27 Jan 2003, Robert Schick wrote:
> 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?)
?count.fields

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595