[R] Assigning factor to character vector

[Bert Gunter]

Sorry, I failed to cc: the list.

I also added a slight edit below to clarify my final statement. -- Bert

On Sat, Apr 20, 2013 at 7:17 AM, Bert Gunter <bgunter at gene.com> wrote:
> Milan:
>
> 1. The R Inferno was written by Pat Burns and is not in any way an
> "official" R document. So it is a "Pat Burns" not an "R" issue. You
> can contact him directly, if you wish -- though he monitors this list
> and almost surely has seen this.
>
> 2. EVerything works exactly as documented and expected. See the R
> Language definition ... and perhaps the "Intro to R" tutorial..
>
> Inline comments below.
>
> Cheers,
> Bert
>
> On Sat, Apr 20, 2013 at 5:49 AM, Milan Bouchet-Valat <nalimilan at club.fr> wrote:
>> Hi!
>>
>> Yesterday I accidentally discovered this:
>>> a <- LETTERS[1:5]
>>> a
>> [1] "A" "B" "C" "D" "E"
>
> a is a character vector.
>>>
>>> a[1] <- factor(a[1])
> The RHS is an vector of integers with additional attributes that define a factor
> The replacement of the first element of a, a character vector,  by an
> integer causes the integer to be silently coerced to a character. The
> default S3 replacement method is used -- see ?UseMethod. or the R
> Intro for info on S3 methods
>
>>> a
>> [1] "1" "B" "C" "D" "E"
>>
>> BUT:
>>> b <- factor(LETTERS[1:5])
> b is a factor
>
>>> b
>> [1] A B C D E
>> Levels: A B C D E
>>> b[1] <- factor(b[1])
>>> b
>> [1] A B C D E
>> Levels: A B C D E
>>> b[1] <- as.character(b[1])
> The replacement method for a factor is used in
b[1] <- factor(b[1])
See ?"[<-.factor" .
>
> Cheers,
> Bert
>
> The replacement
>>> b
>> [1] A B C D E
>> Levels: A B C D E
>>
>> I think this would definitely deserve a mention in the R Inferno...
>>
>> I guess this is documented somewhere (though I could not find anything
>> in help("[<-"). Would someone be kind enough to give me the explanation
>> of this behavior? I suspect this has something to do with the coercion
>> order, but I do not really get why a[1] does not get assigned the result
>> of as.character(factor(a[1]))... Probably, there is no special-casing of
>> factors, which are handled as integer vectors?
>>
>> Wouldn't it be useful to print a warning when this happens, since nobody
>> reasonable would rely on such a special behavior? I wish R had a "safe
>> mode" where all these tricky implicit coercion cases would warn... :-/
>>
>>
>> Regards
>>
>> ______________________________________________
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>> 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.
>
>
>
> --
>
> Bert Gunter
> Genentech Nonclinical Biostatistics
>
> Internal Contact Info:
> Phone: 467-7374
> Website:
> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm



-- 

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm