R help - Sep 2012 - Trap an error from a function

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?
 
Thanks,
John
 
 
John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
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On 12-09-18 8:10 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?
You can wrap the call in try().  Then check whether the result inherits 
from try-error, e.g.

res <- try( ... )
if (inherits(res, "try-error")) { do something to handle the error }
else { proceed as you would with no error }

In the example you gave, the problem is that even a sample size of 2 
gives more than 0.8 power.  The function should probably check for that 
case, but it doesn't.

Duncan Murdoch

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