[R] Partial sort?

[Prof Brian Ripley]

1) Unless you know what you are doing, you don't want to use it since 
the speed advantage is small at best.

2) Suppose you want the median of 2n+1 values.  Then all you need is a 
value with n others to the left and n to the right.  That is a 
partially sorted vector.  Example

> set.seed(123)
> x <- round(runif(11), 3)
> sort.int(x, partial=6)
  [1] 0.046 0.288 0.409 0.457 0.528 0.551 0.788 0.892 0.940 0.883 0.957

Thare are 5 values on each side of 0.551, but the ones on the right 
are not sorted.

Now timings:

set.seed(123)
system.time(for(i in 1:1e5) sort.int(round(runif(11), 3)))
set.seed(123)
system.time(for(i in 1:1e5) sort.int(round(runif(11), 3), partial=6))

Try it for yourself, but remember this is a normally a tiny part of a 
real problem and that specifying even 10% of the quantiles can be 
slower than a full sort.

On Wed, 14 Jan 2009, rkevinburton at charter.net wrote:

> This is definitely a newbie question but from the documentation I have not been able to figure out what the partial sort option on the sort method does. I have read and re-read the documentation and looked at the examples but for some reason it doesn't register. Would someone attempt to explain what sort with a non-null partial array of indices does?
>
> Thank you.
>
> Kevin
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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