Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com
> -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Peng Yu > Sent: Thursday, December 31, 2009 2:16 PM > To: [email protected] > Subject: [R] How x[, 'colname1'] is implemented? > > I don't see where describes the implementation of '[]'. You'd probably have to look in the source code for implementation details like that. > 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? You can easily run timing tests in R by using system.time(). The sum of the first 2 components of its output gives the CPU time. E.g., > f<-function(ncol){ d<-data.frame(as.list(1:ncol)) names(d)<-paste("Col",1:ncol) sum(system.time(for(i in 1:100)d['Col 1'])[1:2]) } > z <- sapply(n<-2^(0:20), f) > z [1] 0.06 0.01 0.01 0.02 0.00 0.03 [7] 0.02 0.01 0.01 0.03 0.02 0.02 [13] 0.02 0.10 0.16 0.33 0.63 1.49 [19] 3.22 8.35 18.91 > plot(n, z, log="xy") # neither 0(1) nor O(ncol) Compare the results to subscripting by number and see how fast the column name to column number algorithm with various naming schemes. Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
