[Rd] round() and signif() do not check argument names when a single argument is given

[Shane Mueller]

On Fri, May 22, 2020 at 9:55 PM David Winsemius <dwinsemius using comcast.net>
wrote:

> The premise in the first few lines of your preamble is at odds (in the
> logical sense) with my understanding of primitive function behavior. Try:
>
> data.frame(x=1:2,y=letters[1:2])[j=2, i=1]
>
> David
>

I had never seen naming indexes of the [] operator.  The documentation of
[] indicates that it does argument matching in a non-standard way,
recommends against doing it, and states the [.data.frame behavior used in
this example is 'undocumented'.  In the example above a warning is thrown
as well.

Here is the [] documentation:

Argument matching
> Note that these operations do not match their index arguments in the
> standard way: argument names are ignored and positional matching only is
> used. So m[j = 2, i = 1] is equivalent to m[2, 1] and not to m[1, 2].
>
> This may not be true for methods defined for them; for example it is not
> true for the data.frame methods described in [.data.frame which warn if i
> or j is named and have undocumented behaviour in that case.
>
> To avoid confusion, do not name index arguments (but drop and exact must
> be named).
>


For the data frames operator [], i and j appear to be named and used
arguments, as the following causes an unused argument error for k:
data.frame(x=1:2,y=letters[1:2])[j=2, k=1]

The analog for round() would be indexing with something like  [k=1,] alone,
which causes an unused argument error error for data frames, which is what
I'm suggesting round(banana=3.5) should do.   (note it works for matrix as
documented).

Best,
Shane

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