R help - Apr 2013 - Factor to numeric conversion - as.numeric(as.character(f))[f] - Language definition seems to say to not use this.

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.

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Hi Mathew,

In what way are they at odds?

On Mon, Apr 1, 2013 at 1:48 PM, Matthew Lundberg
<matthew.k.lundberg at gmail.com> 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.
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>