Trap an error from a function

Instead of "someClass" %in% class(someThing), as in
 function(x)  if( "try-error" %in%
                     class( try( test[test[x:1]] ) ) ){
                     2}else{0}
it is better to use inherits(someThing, "someClass"), as in
   function(x) {
             res <- try( test[test[x:1]], silent=TRUE )
             if (inherits(res, "try-error")) { NA } else { res }}
(where I return the result of the attempted operation instead of 2
and NA for an error instead of 0.)

A related approach is to use tryCatch():
    function(x) tryCatch(test[test[x:1]], error=function(e)NA)
You can also catch warnings and messages with tryCatch.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
Of David Winsemius
Sent: Tuesday, September 18, 2012 5:35 PM
To: John Sorkin
Cc: r-help at r-project.org
Subject: Re: [R] Trap an error from a function

On Sep 18, 2012, at 5:10 PM, John Sorkin wrote:

Window 7
R 2.15

I am writing a simulation which generates sample sized estimates from simulated data.
When I run the function shown below,
power.t.test(delta=14.02528,sd=1.945226,power=0.8,sig.level=0.05)

I get an error message:

power.t.test(delta=14.02528,sd=1.945226,power=0.8,sig.level=0.05)
Error in uniroot(function(n) eval(p.body) - power, c(2, 1e+07)) :
 f() values at end points not of opposite sign

The fact that the function can not return a sample size is OK, however I need to trap
the error and set the sample size equal to NA. How do I trap the error so that when the
error occurs I can set sample size equal to NA?

?conditions   #### has lots of fancy stuff
# But I use just plain old `try`

test=-10:10
sapply(test, function(x)  if( "try-error" %in%
                     class( try( test[test[x:1]] ) ) ){
                     2}else{0} )
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
Error in test[x:1] : only 0's may be mixed with negative subscripts
 [1] 2 2 2 2 2 2 2 2 2 2 0 0 0 0 0 0 0 0 0 0 0

You still get the messages but the code runs.

--

David Winsemius, MD
Alameda, CA, USA

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Trap an error from a function

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