[R] Is a list an atomic object? (or is there an issue with the help page of ?tapply ?)

[Hervé Pagès]

You could also call this "interesting example" a bug.

Clearly not enough code reuse in the implementation of tapply().
Instead of the current 25 lines of code, it could be a simple
wrapper around split() and sapply() e.g.. something like:

   tapply2 <- function(X, INDEX, FUN=NULL, ..., simplify=TRUE)
   {
     f <- make_factor_from_INDEX(INDEX)  # same as tapply(INDEX=INDEX, 
FUN=NULL)
     sapply(split(X, f), FUN, ..., simplify=simplify, USE.NAMES=FALSE)
   }

and then be guaranteed to behave consistently with split() and
sapply(). Also the make_factor_from_INDEX() step maybe could be
shared with what aggregate.data.frame() does internally with its
'by' argument.

Still a mystery to me why the power of code sharing/reuse is so
often underestimated :-/

H.

On 02/15/2017 11:32 AM, Hadley Wickham wrote:
> It seems like this should be consistent with split(), since that's
> what actually powers the behaviour.
>
> Reading the description for split leads to this rather interesting example:
>
> tapply(mtcars, 1:11, I)
>
> Hadley
>
> On Tue, Feb 14, 2017 at 7:10 PM, Hervé Pagès <hpages at fredhutch.org> wrote:
>> Hi,
>>
>> tapply() will work on any object 'X' that has a length and supports
>> single-bracket subsetting. These objects are sometimes called
>> "vector-like" objects. Atomic vectors, lists, S4 objects with a "length"
>> and "[" method, etc... are examples of "vector-like" objects.
>>
>> So instead of saying
>>
>>   X: an atomic object, typically a vector.
>>
>> I think it would be more accurate if the man page was saying something
>> like
>>
>>   X: a vector-like object that supports subsetting with `[`, typically
>>      an atomic vector.
>>
>> H.
>>
>>
>> On 02/04/2017 04:17 AM, Tal Galili wrote:
>>>
>>> In the help page of ?tapply it says that the first argument (X) is "an
>>> atomic object, typically a vector."
>>>
>>> However, tapply seems to be able to handle list objects. For example:
>>>
>>> ###################
>>>
>>> l <- as.list(1:10)
>>> is.atomic(l) # FALSE
>>> index <- c(rep(1,5),rep(2,5))
>>> tapply(l,index,unlist)
>>>
>>>> tapply(l,index,unlist)
>>>
>>> $`1`
>>> [1] 1 2 3 4 5
>>>
>>> $`2`
>>> [1]  6  7  8  9 10
>>>
>>>
>>> ###################
>>>
>>> Hence, does it mean a list an atomic object? (which I thought it wasn't)
>>> or
>>> is the help for tapply needs updating?
>>> (or some third option I'm missing?)
>>>
>>> Thanks.
>>>
>>>
>>>
>>>
>>>
>>> ----------------Contact
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>>> PLEASE do read the posting guide
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>>>
>>
>> --
>> Hervé Pagès
>>
>> Program in Computational Biology
>> Division of Public Health Sciences
>> Fred Hutchinson Cancer Research Center
>> 1100 Fairview Ave. N, M1-B514
>> P.O. Box 19024
>> Seattle, WA 98109-1024
>>
>> E-mail: hpages at fredhutch.org
>> Phone:  (206) 667-5791
>> Fax:    (206) 667-1319
>>
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> 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.
>
>
>

-- 
Hervé Pagès

Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M1-B514
P.O. Box 19024
Seattle, WA 98109-1024

E-mail: hpages at fredhutch.org
Phone:  (206) 667-5791
Fax:    (206) 667-1319