On Fri, Jan 1, 2010 at 6:52 AM, Barry Rowlingson <b.rowling...@lancaster.ac.uk> wrote: > On Thu, Dec 31, 2009 at 11:27 PM, Peng Yu <pengyu...@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.
I'm not complaining that it is not documented. > 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. > */ Could you explain what ns and nx represent? > 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) ); ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
