[Rd] Associative array?

[Roger Peng]

If I recall correctly, I thought indexing a vector/list with a
character vector uses hashing if the vector is over a certain length
(I can't remember the cutoff). Otherwise, it's a linear operation.

-roger

On Thu, Mar 11, 2010 at 8:09 PM, Ben <misc7 at emerose.org> wrote:
>> lists are generic vectors with names so lookup is O(n). Environments
>> in R are true hash tables for that purpose:
>
> Ahh, thanks for the information!  A function I wrote before indexing
> on a data frame was slower than I expected, and now I know why.
>
>> 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.
>
> It's no big problem, it's just that if the solution is to convert to
> character type, then there are some implementation details to worry
> about.  For instance, I assume that as.character(x) is a reversible
> 1-1 mapping if x is an integer (and not NA or NULL, etc).  But
> apparently that isn't exactly true for floats, and it would get more
> complicated for other data types.  So that's why I said it would not
> be elegant, but that is a very subjective statement.
>
> On a deeper level, it seems counterintuitive to me that indexing in R
> is O(n).  Futhermore, associative arrays are a fundamental data type,
> so I think it's weird that I can read the R tutorial, the R language
> definition, and even the manual page for new.env() and still not have
> enough information to build a decent one.  So IMHO things would be
> better if R had a built-in easy-to-use general purpose associative
> array.
>
>> I don't see a problem thus I'm not surprised it didn't come up
>> ;). But maybe I'm just missing your point ...
>
> Nope, this has come up before---I think R and I are just on different
> wavelengths.  Various things that I think are a problem with R are
> apparently not, and it's fine the way it is.
>
> Anyway, sorry for getting off topic ;-) You posted everything I need to know and I really appreciate your help.
>
>
> --
> Ben Escoto
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



-- 
Roger D. Peng  |  http://www.biostat.jhsph.edu/~rpeng/