Re: [Rd] RFC: adding an 'exact' argument to [[

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Tue, 22 May 2007 13:23:39 -0400

In addition to $ that was mentioned in this thread there is also attr, e.g.

> names(attributes(CO2))

[1] "names" "row.names" "class" "formula" "outer" "labels" [7] "units"
> attr(CO2, "f") # matches "formula"

uptake ~ conc | Plant

On 5/17/07, Seth Falcon <sfalcon_at_fhcrc.org> wrote:
> Hi all,
>
> One of the things I find most problematic in R is the partial matching
> of names in lists. Robert and I have discussed this and we believe
> that having a mechanism that does not do partial matching would be of
> significant benefit to R programmers. To that end, I have written a
> patch that modifies the behavior of "[[" as follows:
>
> 1. [[ gains an 'exact' argument with default value NA
>
> 2. Behavior of 'exact' argument:
>
> exact=NA
> partial matching is performed as usual, however, a warning
> will be issued when a partial match occurs. This is the
> default.
>
> exact=TRUE
> no partial matching is performed.
>
> exact=FALSE
> partial matching is allowed and no warning issued if it
> occurs.
>
> This change has been discussed among R-core members and there appeared
> to be a general consensus that this approach was a good way to
> proceed. However, we are interested in other suggestions from the
> broader R developer community.
>
> Some additional rationale for our approach:
>
> Lists are used as the underlying data structures in many R programs
> and in these cases the named elements are not a fixed set of things
> with a fixed set of names. For these programs, [[ will be used with
> an argument that gets evaluated at runtime and partial matching here
> is almost always a disaster. Furthermore, dealing with data that has
> common prefixes happens often and is not an exceptional circumstance
> (a precondition for partial matching issues).
>
> We have tested a similar patch that simply eliminated partial matching
> for [[ on all CRAN and Bioconductor packages and did not see any
> obvious failures.
>
> A downside of this approach is that S4 methods on [[ will need to be
> modified to accommodate the new signature. However, by adding an
> argument, we are able to move more slowly towards a non-partially
> matching [[ (eventually, the default could be exact=TRUE, but that is
> a discussion for another day).
>
>
> + seth
>
> --
> Seth Falcon | Computational Biology | Fred Hutchinson Cancer Research Center
> http://bioconductor.org
>
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-devel
>



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