Re: [Rd] How x[, 'colname1'] is implemented?

From: Barry Rowlingson <b.rowlingson_at_lancaster.ac.uk>
Date: Fri, 01 Jan 2010 12:52:26 +0000

On Thu, Dec 31, 2009 at 11:27 PM, Peng Yu <pengyu.ut_at_gmail.com> wrote:
> I don't see where describes the implementation of '[]'.
>
> For example, if x is a matrix or a data.frame, how the lookup of
> 'colname1' is x[, 'colname1'] executed. Does R perform a lookup in the
> a hash of the colnames? Is the reference O(1) or O(n), where n is the
> second dim of x?

 Where have you looked? I doubt this kind of implementation detail is in the .Rd documentation since a regular user doesn't care for it.

 As Obi-wan Kenobi may have said in Star Wars: "Use the source, Luke!":

 Line 450 of subscript.c of the source code of R 2.10 is the stringSubscript function. It has this comment:

/* The original code (pre 2.0.0) used a ns x nx loop that was too
 * slow.  So now we hash.  Hashing is expensive on memory (up to 32nx
 * bytes) so it is only worth doing if ns * nx is large.  If nx is
 * large, then it will be too slow unless ns is very small.
 */

The definition of "large" and "small" here appears to be such that:

457: Rboolean usehashing = in && ( ((ns > 1000 && nx) || (nx > 1000 && ns)) || (ns * nx > 15*nx + ns) );

  Barry



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