[R] Indexing Lists and Partial Matching

[McGehee, Robert]

Thank you both for your reference. I had missed the previous discussions
before posting.

I am surprised to hear that there is code that relies on this indexing
behavior, especially if it is in the base package. I'm not sure how a
function could even make use of this feature without first asking R what
the names of the list or data frame are, and then intentionally
shortening them to something else. It even seems reasonable that if code
_does_ rely on this behavior, then it may be subject to other problems
anyway, such as if the wrong data is unintentionally returned (when NULL
or error should be returned instead). (Although I freely acknowledge my
ignorance of the uses of this feature as I only recently discovered it.)

>From the previous posts, it seems the only way in R to code around this
is to _always_ check the names of a list before indexing, as anything
else could lead to very subtle errors in complex code, unless one can a
priori guarantee that the list names are always distinguishable. 

Perhaps one easy way to optionally remove this feature without breaking
anything would be to have an option/flag in the description or namespace
of a package indicating that list-indexing partial-matching should not
be used for any function within that package. But that might be a bit
hackish.

However, for my personal code, the a[[match("abc", names(a))]] construct
(from one of the Nov 18th posts) is easy enough to use, so no intention
to rehash an already well-discussed topic.

Thanks,
Robert

PS. None of this applies to partial matching of function arguments, as
this is certainly widely used.

-----Original Message-----
From: Huntsinger, Reid [mailto:reid_huntsinger at merck.com] 
Sent: Thursday, January 27, 2005 10:15 AM
To: 'McGehee, Robert'; r-help at stat.math.ethz.ch
Subject: RE: [R] Indexing Lists and Partial Matching


This came up a few months ago. Check the thread on hashing and partial
matching around Nov 18. The short answer is no, you can't turn it off
because lots of code relies on that behavior. 

Reid Huntsinger 

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of McGehee, Robert
Sent: Thursday, January 27, 2005 9:34 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Indexing Lists and Partial Matching


I was unaware until recently that partial matching was used to index
data frames and lists. This is now causing a great deal of problems in
my code as I sometimes index a list without knowing what elements it
contains, expecting a NULL if the column does not exist. However, if
partial matching is used, sometimes R will return an object I do not
want. My question, is there an easy way of getting around this?

For example:
> a <- NULL
> a$abc <- 5
> a$a
[1] 5
> a$a <- a$a
> a
$abc
[1] 5
$a
[1] 5

Certainly from a coding prospective, one might expect assigning a$a to
itself wouldn't do anything since either 1) a$a doesn't exist, so
nothing happens, or 2) a$a does exist and so it just assigns its value
to itself. However, in the above case, it creates a new column entirely
because I happen to have another column called a$abc. I do not want this
behavior.

The solution I came up with was to create another indexing function that
uses the subset() (which doesn't partial match), then check for an
error, and if there is an error substitute NULL (to mimic the "["
behavior). However, I don't really want to start using another indexing
function altogether just to get around this behavior. Is there a better
way? Can I turn off partial matching?

Thanks,
Robert


Robert McGehee
Geode Capital Management, LLC
53 State Street, 5th Floor | Boston, MA | 02109
Tel: 617/392-8396    Fax:617/476-6389
mailto:robert.mcgehee at geodecapital.com



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