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

From: Bill Dunlap <bill_at_insightful.com>
Date: Thu, 17 May 2007 11:54:45 -0700 (PDT)

On Thu, 17 May 2007, Seth Falcon wrote:

> 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).

This sounds interesting. Do you intend to leave the $ operator alone, so it will continue to do partial matching? I suspect that that is where the majority of partial matching for list names is done. It might be nice to have an option that made x$partial warn so we would fix code that relied on partial matching, but that is lower priority.



Bill Dunlap
Insightful Corporation
bill at insightful dot com
360-428-8146

 "All statements in this message represent the opinions of the author and do  not necessarily reflect Insightful Corporation policy or position."



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