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-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
> Behalf Of Bert Gunter
> Sent: Thursday, July 19, 2012 9:11 AM
> To: Hadley Wickham
> Cc: r-help@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 <had...@rice.edu> wrote:
> > On Thu, Jul 19, 2012 at 8:02 AM, Jan van der Laan <rh...@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@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-
> biostatistics/pdb-ncb-home.htm
>
> ______________________________________________
> R-help@r-project.org mailing list
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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