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

[Andrew Yee]

Thank you very much for your suggestion...that works perfectly.

Thanks,
Andrew

On Mon, Apr 4, 2011 at 5:46 PM, Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
> 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
>