[R] how to handle no lines in input with pipe()

[Prof Brian Ripley]

On Mon, 4 Apr 2011, Andrew Yee wrote:

> This has to do with using pipe() and grep and read.csv()
>
> I have a .csv file that I grep using pipe() and read.csv() as follows:
>
> read.csv(pipe('grep foo bar.csv'))
>
> However, is there a way to have this command run when for example,
> there is no "foo" text in the bar.csv file?  I get an error message
> (appropriately):
>
> Error in read.table(file = file, header = header, sep = sep, quote = quote,  :
>  no lines available in input
>
> Is there a way to "inspect" the output of pipe before passing it on to
> read.csv()?

You have to read from a pipe to 'inspect' it.  So

tmp <- readLines(pipe('grep foo bar.csv'))
if(!length(tmp)) do something else
else {
   res <- read.csv(con <- textConnection(tmp))
   close(con)
}

OTOH, unless the file is enormous you could simply read it into R and 
use grep(value = TRUE) on the character vector.

>
> Thanks,
> Andrew
>
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-- 
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