[R] Factor to numeric conversion - as.numeric(as.character(f))[f] - Language definition seems to say to not use this.

[Bert Gunter]

Yup. Note also:

> as.character.factor
function (x, ...)
levels(x)[x]

But of course this is OK, since this can change if the implementation
does. Which is the whole point, of course.

-- Bert



On Mon, Apr 1, 2013 at 12:16 PM, Matthew Lundberg
<matthew.k.lundberg at gmail.com> wrote:
>
> When used as an index, the factor is implicitly converted to integer.  In
> the expression as.numeric(levels(f))[f], the vector as.numeric(levels(f))
> is indexed by as.integer(f).
>
> This appears to rely on the current implementation, as mentioned in section
> 2.3.1 of the language definition.
>
>
> On Mon, Apr 1, 2013 at 1:49 PM, Peter Ehlers <ehlers at ucalgary.ca> wrote:
>
> > On 2013-04-01 10:48, Matthew Lundberg wrote:
> >
> >> These two seem to be at odds.  Is this the case?
> >>
> >>  From help(factor) - section Warning:
> >>>
> >>
> >> To transform a factor f to approximately its original numeric values,
> >> as.numeric(levels(f))[f] is recommended and slightly more efficient than
> >> as.numeric(as.character(f)).
> >>
> >>  From the language definition - section 2.3.1:
> >>>
> >>
> >> Factors are currently implemented using an integer array to specify the
> >> actual levels and
> >> a second array of names that are mapped to the integers. Rather
> >> unfortunately users often
> >> make use of the implementation in order to make some calculations easier.
> >> This, however,
> >> is an implementation issue and is not guaranteed to hold in all
> >> implementations of R.
> >>
> >
> > Hint:
> >
> >  f <- factor(sample(5, 10, TRUE))
> >  as.numeric(levels(f))[f]
> >
> >  g <- factor(sample(letters[1:5], 10, TRUE))
> >  as.numeric(levels(g))[g]
> >
> > Peter Ehlers
> >
> >
> >
> >>         [[alternative HTML version deleted]]
> >>
> >> ______________________________**________________
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> >> posting-guide.html <http://www.R-project.org/posting-guide.html>
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >>
> >
>
>         [[alternative HTML version deleted]]
>
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--

Bert Gunter
Genentech Nonclinical Biostatistics

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