[R] The try() function with read.delim().

[Steven McKinney]

> -----Original Message-----
> From: r-help-bounces at r-project.org on behalf of Rolf Turner
> Sent: Tue 5/13/2008 6:12 PM
> To: R help forum
> Subject: [R] The try() function with read.delim().
>  
> 
> I have written a function which reads data from files using read.delim 
> ().
> The names of these files are complicated and are built using arguments
> to the wrapper function.  Since the files in question may or may not
> exist, I thought to enclose the read.delim() call in a try():
> 
> 	file <- <complicated expression>
> 	temp <- try(read.delim(file))
> 	if(inherits(temp,"try-error")) {
> 		(Do something)
> 	} else {
> 		(Do something else)
> 	}
> 
> But this doesn't work.  If file doesn't exist, then I get an error,
> and the wrapper function terminates, despite the presence of the try().
> The object ``temp'' never gets created.
> 
> I can of course easily get around the problem by testing with
> 
> 	file.exists(file)
> 
> I just wondered --- why isn't try() working as expected here?  Am I  
> being
> silly to expect it to work?  If so, what am I not understanding about  
> try()?

It is working for me using R-2.7.0 on a Mac running OS X 10.4
and it also runs pretty much as shown below on my Windoze machine
(R-2.7.0 on Windows XP)

So you may be encountering a bug - not sure what your
sessionInfo() etc. is.

Here's my output of your example

> file <- "foo.bar"
> file.exists(file)
[1] FALSE
> temp <- try(read.delim(file))
Warning in file(file, "r") :
  cannot open file 'foo.bar': No such file or directory
Error in file(file, "r") : cannot open the connection
> temp
[1] "Error in file(file, \"r\") : cannot open the connection\n"
attr(,"class")
[1] "try-error"
> rm("temp")
> temp
Error: object "temp" not found
> temp <- try(read.delim(file))
Warning in file(file, "r") :
  cannot open file 'foo.bar': No such file or directory
Error in file(file, "r") : cannot open the connection
> temp
[1] "Error in file(file, \"r\") : cannot open the connection\n"
attr(,"class")
[1] "try-error"
> class(temp)
[1] "try-error"
> sessionInfo()
R version 2.7.0 (2008-04-22) 
powerpc-apple-darwin8.10.1 

locale:
en_CA.UTF-8/en_CA.UTF-8/C/C/en_CA.UTF-8/en_CA.UTF-8

attached base packages:
[1] tcltk     splines   stats     graphics  grDevices utils     datasets 
[8] methods   base     

other attached packages:
[1] relimp_0.9-8      abind_1.1-0       rpart_3.1-41      Rcmdr_1.3-14     
[5] car_1.2-8         lme4_0.999375-14  Matrix_0.999375-9 lattice_0.17-6   
[9] survival_2.34-1  

loaded via a namespace (and not attached):
[1] grid_2.7.0  tools_2.7.0
> 

Best

Steve McKinney

> 
> Thanks for any insights.
> 
> 	cheers,
> 
> 		Rolf
> 
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