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
>>
>> ______________________________________________
>> 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.
>
>
>
> --
>
> 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