[R] all(logical(0)) and any(logical(0))

[Liaw, Andy]

Thanks to Rolf, Thomas, Duncan & Doug for the explanations!  It's one of
those things that I should have remembered from high school but clearly
didn't...

I've changed my code to:

[If y is factor:]
        if (!is.null(ytest)) {
            if (!is.factor(ytest)) stop("ytest must be a factor")
            if (!all(levels(y) == levels(ytest)))
                stop("y and ytest must have the same levels")
        }

For Duncan: `y' can be either factor or numeric.  That's how randomForest
determines whether it's a classification or regression problem.

Thanks again to all!

Andy

> From: Liaw, Andy
> 
> Dear R-help,
> 
> I was bitten by the behavior of all() when given logical(0):  
> It is TRUE!
> (And any(logical(0)) is FALSE.)  Wouldn't it be better to 
> return logical(0)
> in both cases?
> 
> The problem surfaced because some un-named individual called 
> randomForest(x,
> y, xtest, ytest,...), and gave y as a two-level factor, but 
> ytest as just
> numeric vector.  I thought I check for that in my code by testing for
> 
> if (!all(levels(y) == levels(ytest))) stop(...)
> 
> but levels() on a non-factor returns NULL, and the comparison 
> ended up being
> logical(0).  Since all(logical(0)) is TRUE, the error is not flagged.
> 
> Best,
> Andy
> 
> Andy Liaw, PhD
> Biometrics Research      PO Box 2000, RY33-300     
> Merck Research Labs           Rahway, NJ 07065
> mailto:andy_liaw at merck.com        732-594-0820
> 
> 
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