[Rd] split() - unexpected sorting of results

[Hervé Pagès]

Hi,

On 10/20/2017 12:53 PM, Peter Meissner wrote:
> Thanks, for the explanation.
>
> Still, I think this is surprising bahaviour which might be handled better.

Maybe a little surprising, but no more than:

 > x <- sample(11L)

 > sort(x)
  [1]  1  2  3  4  5  6  7  8  9 10 11

 > sort(as.character(x))
  [1] "1"  "10" "11" "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9"

The fact that sort(), as.factor(), split() and many other things behave
consistently with respect to the underlying order of character vectors
avoids other even bigger surprises.

Also note that the underlying order of character vectors actually
depends on your locale. One way to guarantee consistent results across
platforms/locales is by explicitly specifying the levels when making
a factor e.g.

   f <- factor(x, levels=unique(x))
   split(1:11, f)

This is particularly sensible when writing unit tests.

Cheers,
H.

>
> Best, Peter
>
> Am 20.10.2017 9:49 nachm. schrieb "Iñaki Úcar" <i.ucar86 at gmail.com>:
>
>> Hi Peter,
>>
>> 2017-10-20 21:33 GMT+02:00 Peter Meissner <retep.meissner at gmail.com>:
>>> Hey,
>>>
>>> I found this - for me - quite surprising and puzzling behaviour of
>> split().
>>>
>>>
>>> split(1:11, as.character(1:11))
>>> split(1:11, 1:11)
>>>
>>>
>>> When splitting by numerics everything works as expected - sorting of
>> input
>>> == sorting of output -- but when using a character vector everything gets
>>> re-sorted alphabetical.
>>>
>>>
>>> Although, there are some references in the help files to what happens
>> when
>>> using split, I did not find any note on this - for me - rather unexpected
>>> behaviour.
>>
>> As the documentation states,
>>
>>         f: a ‘factor’ in the sense that ‘as.factor(f)’ defines the
>>            grouping, or a list of such factors in which case their
>>            interaction is used for the grouping.
>>
>> And, in fact,
>>
>>> as.factor(1:11)
>>   [1] 1  2  3  4  5  6  7  8  9  10 11
>> Levels: 1 2 3 4 5 6 7 8 9 10 11
>>
>>> as.factor(as.character(1:11))
>>   [1] 1  2  3  4  5  6  7  8  9  10 11
>> Levels: 1 10 11 2 3 4 5 6 7 8 9
>>
>> Regards,
>> Iñaki
>>
>>> I would like it best when the sorting of split results stays the same no
>>> matter the input (sorting of input == sorting of output)
>>>
>>> If that is not possibly a note of caution in the help pages and maybe an
>>> example might be valuable.
>>>
>>>
>>> Best, Peter
>>>
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>>>
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>
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-- 
Hervé Pagès

Program in Computational Biology
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