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

[Peter Ehlers]

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

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