R help - May 2009 - R errors when trying to read a file

Colleagues,

I have encountered a problem in version 2.9 of R, running in both  
Vista and OS X.  My code is quite lengthy but the critical line is:
	if (file.exists(FILENAME))	readLines(FILENAME))

This triggers the error:
	Error in file(con, "r") : cannot open the connection
	Calls: readLines -> file
	Execution halted

The file that it is reading is being created on the fly, then  
eventually deleted.  The conditional was used to ensure that the  
readLines command would be executed only when the file existed.

Is it possible that the file is deleted AFTER the conditional is  
evaluated but before the readLines command is executed?  If so, is  
there any means to prevent an error condition?

Any help will be greatly appreciated.  Thanks.

Dennis

Dennis Fisher MD
P < (The "P Less Than" Company)
Phone: 1-866-PLessThan (1-866-753-7784)
Fax: 1-415-564-2220
www.PLessThan.com

On Mon, 11 May 2009, Dennis Fisher wrote:
> Colleagues,
>
> I have encountered a problem in version 2.9 of R, running in both Vista and
> OS X.  My code is quite lengthy but the critical line is:
> 	if (file.exists(FILENAME))	readLines(FILENAME))
>
> This triggers the error:
> 	Error in file(con, "r") : cannot open the connection
> 	Calls: readLines -> file
> 	Execution halted
>
> The file that it is reading is being created on the fly, then eventually 
> deleted.  The conditional was used to ensure that the readLines command
would
> be executed only when the file existed.
>
> Is it possible that the file is deleted AFTER the conditional is evaluated 
> but before the readLines command is executed?  If so, is there any means to
> prevent an error condition?
It is possible.  But so are a lot of other things, and your pretest is 
just not enough -- for example another process might have the file 
open for exclusive access (quite common in the Windows world) in which 
case it exists but you cannot open it for reading.

There is only one sure way to find out if you can read a file, and 
that is to try to read it.  So I'd simply use try() or tryCatch() on 
the readLines call.
>
> Any help will be greatly appreciated.  Thanks.
>
> Dennis
>
> Dennis Fisher MD
> P < (The "P Less Than" Company)
> Phone: 1-866-PLessThan (1-866-753-7784)
> Fax: 1-415-564-2220
> www.PLessThan.com
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
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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