[R] Indexing by factor misfeature

Prof Brian Ripley ripley at stats.ox.ac.uk
Tue Apr 20 11:00:41 CEST 2004

This *is* a documented feature: see ?Extract.

I am afraid your `of course' is completely non-obvious to me.  I would 
object strongly to code written to use a documented feature issuing a 

On Tue, 20 Apr 2004, Wilhelm B. Kloke wrote:

> Yesterday I was biten by a feature, which I find too dangerous.
> I wanted to use a factor `Subject´ as index into a data frame, whose row
> names were the levels of this factor. So there a 2 different possible
> interpretations of this: Either Subject is coerced to numeric or to
> character. The intended interpretation was, of course, `as.character(Subject)'.
> R did `as.numeric(Subject)´. This will not cause a semantic error, when
> the levels of Subject happen to be in numeric order; so it will be
> unnoticed quite often.
> I propose to issue a warning whenever a factor is implicitly coerced,
> at least in an ambiguous context.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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