R help - Mar 2004 - how to use conditional statements to handle exceptions?

Hello,

I have a problem to handle the following statements.
  
  for(i in [1:3])
  {
    file=paste("file", i, ".dat")
    bb <- read.table(file)
    x11()
    plot(bb)
    dev.off()

  }
When the input .dat file is empty, the program stops
running and an error message appears. Could someone
tell me how to handle this exception?

Thanks

Take a look at try().

-roger

Susan Lin wrote:
> Hello,
> 
> I have a problem to handle the following statements.
>   
>   for(i in [1:3])
>   {
>     file=paste("file", i, ".dat")
>     bb <- read.table(file)
>     x11()
>     plot(bb)
>     dev.off()
> 
>   }
> When the input .dat file is empty, the program stops
> running and an error message appears. Could someone
> tell me how to handle this exception?
> 
> Thanks
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>

If you mean to put a check in to see if the file exists then something
like this would work:

  for(i in 1:3){
    aFile <- paste("file", i, ".dat", sep = "")
    if(file.exists(aFile) == T){
        bb <- read.table(aFile, header = F)
        x11()
        plot(bb)
        dev.off()
    }
  }


If you mean that you want to check to see that it has data in it you
could ammend the if statement to something like 

if(nrow(aFile) > 0)...


HTH, Andy

On Tue, 9 Mar 2004, Susan Lin wrote:
> I have a problem to handle the following statements.
>   
>   for(i in [1:3])
>   {
>     file=paste("file", i, ".dat")
Don't you mean paste("file", i, ".dat",
sep="") ?  Or are your files
called `file 1 .dat'?
>     bb <- read.table(file)
>     x11()
>     plot(bb)
>     dev.off()
This opens a device, plots and closes it instantly.  Is that what you 
want?  I would have had

par(ask=TRUE)

outside the loop, which will automatically launch as graphics device if 
none is open.
> 
>   }
> When the input .dat file is empty, the program stops
> running and an error message appears. Could someone
> tell me how to handle this exception?
?try

-- 
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

For R v1.8.0 you can use tryCatch() (it's great) to catch objects of
class 'condition' and just print() them (instead of letting them
generate errors) like this:

for(i in 1:3) {
  file <- paste("file", i, ".dat", sep="")
  tryCatch({
    bb <- read.table(file)
    x11()
    plot(bb)
    dev.off()
  }, condition=function(ex) {
    print(ex);
  })
}

As soon as/if an error is generated, tryCatch() will catch it an call
the condition function, which will print it.

Details:
If the file does not exists when calling read.table() both a warning
and an error is generated. Since warnings are now of class
simpleWarning and (stop) errors are of class simpleError and both
these are subclasses (via the abstract classes 'warning' and
'error',
respectively) of class 'condition', which is the root class of all
exception classes, it is best to catch by using 'condition' above. If
one use 'error=...' instead, warnings will still be shown.

Moreover, a shorter version of the above would be to use
'condition=print'.
 

Some further comments:
 1) Your code is not correct, e.g. "[1:3]".
    Please submit example code that is executable!
 2) Did you forget sep="" in the paste line?
 1) avoid using "=" to assign; or at least be consistent.

Cheers

Henrik Bengtsson
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Susan Lin
> Sent: den 9 mars 2004 18:12
> To: r-help at stat.math.ethz.ch
> Subject: [R] how to use conditional statements to handle exceptions?
> 
> 
> Hello,
> 
> I have a problem to handle the following statements.
>   
>   for(i in [1:3])
>   {
>     file=paste("file", i, ".dat")
>     bb <- read.table(file)
>     x11()
>     plot(bb)
>     dev.off()
> 
>   }
> When the input .dat file is empty, the program stops
> running and an error message appears. Could someone
> tell me how to handle this exception?
> 
> Thanks
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list 
> https://www.stat.math.ethz.ch/mailma> n/listinfo/r-help
> PLEASE 
> do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>