[R] complexity of operations in R

[William Dunlap]

Preallocation of lists does speed things up.  The following shows
time quadratic in size when there is no preallocation and linear
growth when there is, for size in the c. 10^4 to 10^6 region:
> f <- function(n, preallocate) { v <- if(preallocate)vector("list",n) else list() ; for(i in seq_len(n)) v[[i]] <- i ; v }
> identical(f(17,pre=TRUE), f(17,pre=FALSE))
[1] TRUE
> system.time(f(n=1e4, preallocate=FALSE))
   user  system elapsed
  0.324   0.000   0.326
> system.time(f(n=2e4, preallocate=FALSE)) # 2x n, 4x time
   user  system elapsed
  1.316   0.012   1.329
> system.time(f(n=4e4, preallocate=FALSE)) # ditto
   user  system elapsed
  5.720   0.028   5.754
>
> system.time(f(n=1e4, preallocate=TRUE))
   user  system elapsed
  0.016   0.000   0.017
> system.time(f(n=2e4, preallocate=TRUE)) # 2x n, 2x time
   user  system elapsed
  0.032   0.004   0.036
> system.time(f(n=4e4, preallocate=TRUE)) # ditto
   user  system elapsed
  0.068   0.000   0.069
>
> system.time(f(n=4e5, preallocate=TRUE)) # 10x n, 10x time
   user  system elapsed
  0.688   0.000   0.688

Above 10^6 there is some superlinearity
> system.time(f(n=4e6, preallocate=TRUE)) # 10x n, 20x time
   user  system elapsed
 11.125   0.052  11.181

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
> Behalf Of Bert Gunter
> Sent: Thursday, July 19, 2012 9:11 AM
> To: Hadley Wickham
> Cc: r-help at r-project.org
> Subject: Re: [R] complexity of operations in R
> 
> Hadley et. al:
> 
> Indeed. And using a loop is a poor way to do it anyway.
> 
> v <- as.list(rep(FALSE,dotot))
> 
> is way faster.
> 
> -- Bert
> 
> On Thu, Jul 19, 2012 at 8:50 AM, Hadley Wickham <hadley at rice.edu> wrote:
> > On Thu, Jul 19, 2012 at 8:02 AM, Jan van der Laan <rhelp at eoos.dds.nl> wrote:
> >> Johan,
> >>
> >> Your 'list' and 'array doubling' code can be written much more efficient.
> >>
> >> The following function is faster than your g and easier to read:
> >>
> >> g2 <- function(dotot) {
> >>   v <- list()
> >>   for (i in seq_len(dotot)) {
> >>     v[[i]] <- FALSE
> >>   }
> >> }
> >
> > Except that you don't need to pre-allocate lists...
> >
> > Hadley
> >
> > --
> > Assistant Professor / Dobelman Family Junior Chair
> > Department of Statistics / Rice University
> > http://had.co.nz/
> >
> > ______________________________________________
> > R-help at r-project.org 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.
> 
> 
> 
> --
> 
> Bert Gunter
> Genentech Nonclinical Biostatistics
> 
> Internal Contact Info:
> Phone: 467-7374
> Website:
> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-
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> 
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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