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

[Martin Maechler]

>>>>> Hervé Pagès <hpages at fredhutch.org>
>>>>>     on Tue, 14 Feb 2017 17:10:05 -0800 writes:

    > 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.

Thank you, Hervé!

Actually (someone else mentioned ?)
only   length(X) and  split(X, <group>)   need to work,
and as split() itself is an S3 generic function,  X can be even
more general... well depending on how exactly you understand
"vector-like".

So I would go with

       X: an R object for which a ‘split’ method exists.  Typically
          vector-like, allowing subsetting with ‘[’.


Martin


    > 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.