Ordering long vectors
On Sun, 8 Jun 2003, Thomas Lumley wrote:
On Sat, 7 Jun 2003, [ISO-8859-1] G?ran Brostr?m wrote:
I need to order a long vector of integers with rather few unique values. This is very slow:
I think the culprit is
src/main/sort.c: orderVector1
/* Shell sort isn't stable, but it proves to be somewhat faster
to run a final insertion sort to re-order runs of ties when
comparison is cheap.
*/
This also explains:
aa<-sample(rep(1:10,50000)) system.time( order(aa, 1:length(aa)))
[1] 3.67 0.01 3.68 0.00 0.00
system.time( order(aa))
^C Timing stopped at: 49.33 0.01 49.34 0 0 which is perhaps the simplest work-around :).
Thanks. This is really surprising: it is *much* faster to break ties by a second condition than not breaking them. I think it should be mentioned in the help. And could 'order/sort' be modified to check for 'tieness'? But I guess the the overhead would be too heavy. (if (length(unique(x)) < alpha * length(x)) then .... else ....) G?ran