[R] Problem with modification of [.factor

[David James]

Frank E Harrell Jr wrote:
> Under
> 
> platform i586-pc-linux-gnu
> arch     i586             
> os       linux-gnu        
> system   i586, linux-gnu  
> status                    
> major    1                
> minor    5.0    [1.5.1 not available for Mandrake 8.1]    
> year     2002             
> month    04               
> day      29               
> language R                
> 
> when I modify [.factor to change the default drop=FALSE to drop=TRUE as such:
> 
> '[.factor' <- function(x, i, drop=TRUE) {
>     y <- NextMethod("[")
>     class(y) <- class(x)
>     attr(y, "contrasts") <- attr(x, "contrasts")
>     attr(y, "levels") <- attr(x, "levels")
>     if (drop) 
>         factor(y)
>     else y
> }

A simple fix is 'x <- unclass(x)' prior to NextMethod().  But before
doing this, I'd consider Prof. Ripley's comments on this topic extremely 
carefully.

> 
> I get a recursion error:
> 
> > x <- factor(1:2,1:2,c('a','b'))
> > x[]
> 
> Error in inherits(x, "factor") : evaluation is nested too deeply: infinite recursion?
> 
> If anyone has an explanation/fix I would appreciate getting it.
> 
> Thanks in advance,  -Frank
> -- 
> Frank E Harrell Jr              Prof. of Biostatistics & Statistics
> Div. of Biostatistics & Epidem. Dept. of Health Evaluation Sciences
> U. Virginia School of Medicine  http://hesweb1.med.virginia.edu/biostat
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-- 
David A. James
Statistics Research, Room 2C-253            Phone:  (908) 582-3082       
Bell Labs, Lucent Technologies              Fax:    (908) 582-3340
Murray Hill, NJ 09794-0636
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