Ben,
On Mar 11, 2010, at 8:52 , Ben wrote:
Hi, can someone tell me how to use associative arrays in R? It can
be a hashtable or some kind of tree, as long as the lookups aren't
O(n).
One way to do this is to use names, e.g. in:
list(a=3, ...)[["a"]]
presumably looking up "a" is very quick. (Can someone tell me
offhand how that is implemented? Hashtable?)
lists are generic vectors with names so lookup is O(n). Environments
in R are true hash tables for that purpose:
> h=new.env(hash=TRUE)
> h[["foo"]]="bar"
> ls(h)
[1] "foo"
However, if I wanted to, say,
memoize a numeric function, I can't elegantly use R names because R
names must be characters.
I don't quite understand - characters are (after raw vectors) the most
expressive data type, so I'm not quite sure why that would be a
limitation .. You can cast anything (but raw vector with nulls) into
to a character.
I found the hash package on CRAN:
http://cran.r-project.org/web/packages/hash/index.html
but it seems the keys are still characters. Also, I haven't heard
anyone talking about it. Trees and hashtables are common data
structures, so this problem must come up a lot.
I don't see a problem thus I'm not surprised it didn't come up ;). But
maybe I'm just missing your point ...
Cheers,
Simon
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