warning inside loop

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of Rense
Sent: Monday, January 11, 2010 3:07 PM
To: r-help at r-project.org
Subject: [R] warning inside loop


Hi,

I'm running some data simulations using (mixed effects)* 
regression models
that show difficulty to converge. Therefore, I seek a way of capturing
warnings (of false convergence) inside a loop.

Inside that loop, I modify data and estimate a model. I do so 
many times
with slightly different modifications of the data. Next, I 
extract some of
the model parameters and store these in a matrix. However, as 
some of the
models do not converge well, some of the stored parameters 
are extracted
from the ill-converged models. Therefore, I seek a way of 
automatically
detecting whether the estimation procedure has resulted in a 
warning, so I
can distinguish between the well- and ill-converged models.

I have been trying to use functions as warnings(), as well as 
using the
object last.warning, but unfortunately to no avail.

Try withCallingHandlers(), as in the following function
with returns the value of the expression along with
any warning messages as a list:

withWarnings

function (expr) 
{
    warnings <- character()
    retval <- withCallingHandlers(expr, warning = function(ex) {
        warnings <<- c(warnings, conditionMessage(ex))
        invokeRestart("muffleWarning")
    })
    list(Value = retval, Warnings = warnings)
}
<environment: R_GlobalEnv>

Typical usage would be:

lapply(-1:1, function(i)withWarnings(log(i)))

[[1]]
[[1]]$Value
[1] NaN

[[1]]$Warnings
[1] "NaNs produced"


[[2]]
[[2]]$Value
[1] -Inf

[[2]]$Warnings
character(0)


[[3]]
[[3]]$Value
[1] 0

[[3]]$Warnings
character(0)

Perhaps there is some encapsulation of this already in some
package, as try() encapsulates error catching.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

Although I cannot provide a reproducible example, I 
schematically represent
the procedure I seek to use below:


for (i in 1:10)
{
<<modify data>>
<<estimate model>>

<<<evaluate whether estimation produced warning>>>

<<extract model parameters, and store whether warning occured>>
}

I hope any one can give some guidelines on how to deal with 
warnings inside
a loop.

With Kind regards,

Rense





*Although I use the lme4 package for that actual analysis, I sent my
question to this mailinglist (instead of the R mixed list) 
because I believe
this is a general issue, rather than one associated 
exclusively with mixed
models.
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