[R] Confusing concept of vector and matrix in R

[Patrick Burns]

Two points:

the 'drop' argument is not unrecognized,
just superfluous.

There are one-dimensional cases where the
'drop' argument is not superfluous:

factor(letters)[1:5, drop=TRUE]
factor(letters)[1:5, drop=FALSE]

So my guess is that it would be an exceptional
amount of work to get such a warning correct,
and would possibly cause a substantial performance
hit in one of the most used functions in the
language.

On 27/04/2010 03:36, Stuart Andrews wrote:
>
> Thanks Charles, for clarifying.
>
> My statement holds for matrices, which are 2 dimensional. And, as you
> mentioned, a single index implies vector indexing where the drop
> argument doesn't make sense. I am somewhat relieved, given this new
> understanding.
>
> But I am still puzzled as to why R doesn't complain about the unused
> "drop=F" argument. Since this argument is nonsensical, R should tell me
> this, no? For example, when I add an unrecognized argument to the ls()
> function I get the following:
>
>  > ls(nonsense="42")
> Error in ls(nonsense = "42") : unused argument(s) (nonsense = "42")
>
> Cheers,
> - Stu
>
>
> On Apr 26, 2010, at 9:40 PM, Charles C. Berry wrote:
>
>> On Mon, 26 Apr 2010, Stu wrote:
>>
>>> Hi all,
>>>
>>> One subtlety is that the drop argument only works if you specify 2 or
>>> more indices e.g. [i, j, ..., drop=F]; but not for a single index e.g
>>> [i, drop=F].
>>
>> Wrong.
>>
>>> a <- structure(1:5,dim=5)
>>> dim(a)
>> [1] 5
>>> dim(a[2:3,drop=F]) # don't drop regardless
>> [1] 2
>>> dim(a[2,drop=F]) # dont' drop regardless
>> [1] 1
>>> dim(a[2:3,drop=T]) # no extent of length 1
>> [1] 2
>>> dim(a[2,drop=T]) # drop, extent of length 1
>> NULL
>>
>>
>>>
>>> Why doesn't R complain about the unused "drop=F" argument in the
>>> single index case?
>>
>> In the example you give (one index for a two-dimension array), vector
>> indexing is assumed. For vector indexing, drop is irrelevant.
>>
>> HTH,
>>
>> Chuck
>>>
>>> Cheers,
>>> - Stu
>>>
>>> a = matrix(1:10, nrow=1)
>>> b = matrix(10:1, ncol=1)
>>>
>>> # a1 is an vector w/o dim attribute (i.e. drop=F is ignored silently)
>>> (a1 = a[2:5, drop=F])
>>> dim(a1)
>>>
>>> # a2 is an vector WITH dim attribute: a row matrix (drop=F works)
>>> (a2 = a[, 2:5, drop=F])
>>> dim(a2)
>>>
>>> # b1 is an vector w/o dim attribute (i.e. drop=F is ignored silently)
>>> (b1 = b[2:5, drop=F])
>>> dim(b1)
>>>
>>> # b2 is an vector WITH dim attribute: a column matrix (drop=F works)
>>> (b2 = b[2:5, , drop=F])
>>> dim(b2)
>>>
>>>
>>> On Mar 30, 4:08 pm, lith <minil... at gmail.com> wrote:
>>>>> Reframe the problem. Rethink why you need to keep dimensions. I
>>>>> never ever had to use drop.
>>>>
>>>> The problem is that the type of the return value changes if you happen
>>>> to forget to use drop = FALSE, which can easily turn into a nightmare:
>>>>
>>>> m <-matrix(1:20, ncol=4)
>>>> for (i in seq(3, 1, -1)) {
>>>> print(class(m[1:i, ]))}
>>>>
>>>> [1] "matrix"
>>>> [1] "matrix"
>>>> [1] "integer"
>>>>
>>>> ______________________________________________
>>>> R-h... at r-project.org mailing
>>>> listhttps://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting
>>>> guidehttp://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>> ______________________________________________
>>> 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.
>>>
>>
>> Charles C. Berry (858) 534-2098
>> Dept of Family/Preventive Medicine
>> E mailto:cberry at tajo.ucsd.edu UC San Diego
>> http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
>>
>
> ______________________________________________
> 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.
>

-- 
Patrick Burns
pburns at pburns.seanet.com
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')