[R] Indexing Lists and Partial Matching

[McGehee, Robert]

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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