How x[, 'colname1'] is implemented?

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) );