[R] Selecting elements from a vector (a simple question with weird results)

andrew andrewjohnroyal at gmail.com
Sun Mar 29 11:50:13 CEST 2009


aa[order(aa)] is the same as sort, although sort is much quicker and
has lower memory requirements:

> aa <- rnorm(10000000)
> all(aa[order(aa)] == sort(aa))
[1] TRUE
> system.time(sort(aa))
   user  system elapsed
   7.27    0.08    8.25
> system.time(aa[order(aa)])
   user  system elapsed
  29.58    0.32   31.31

(yes, I have a pitifully slow computer)

HTH,

Andrew.

On Mar 29, 8:01 am, Tal Galili <tal.gal... at gmail.com> wrote:
> Yes Johannes - That helped, thank you.
>
> On Sat, Mar 28, 2009 at 11:50 PM, Johannes Huesing <johan... at huesing.name>wrote:
>
>
>
> > Tal Galili <tal.gal... at gmail.com> [Sat, Mar 28, 2009 at 06:48:36PM CET]:
> > > Hello people.
>
> > > I wish to reorder a simple vector of numbers by another vector of the
> > order
> > > (and then do the same, but with a data frame rows)
>
> > > I try this (which doesn't work) :
> > > > aa <- c(3, 1 ,2 )
> > > > aa[aa]
> > > [1] 2 3 1
>
> > To my mind, it does what you told it to, and therefore "works"
> > in my book. The routine orders the numbers by placing the third element
> > first, the first second, and the second third.
>
> > Maybe aa[order(aa)] does what you mean it to do?
>
> > Best wishes
>
> > Johannes
>
> > --
> > Johannes Hüsing               There is something fascinating about science.
> >                              One gets such wholesale returns of conjecture
> > mailto:johan... at huesing.name  from such a trifling investment of fact.
> >http://derwisch.wikidot.com        (Mark Twain, "Life on the
> > Mississippi")
>
> --
> ----------------------------------------------
>
> My contact information:
> Tal Galili
> Phone number: 972-50-3373767
> FaceBook: Tal Galili
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>
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>
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