[R] General binary search?
Matthew Dowle
mdowle at mdowle.plus.com
Tue Apr 5 10:42:33 CEST 2011
Try data.table:::sortedmatch, which is implemented in C.
It requires it's input to be sorted (and doesn't check)
"Stavros Macrakis" <macrakis at alum.mit.edu> wrote in message
news:BANLkTi=j2LF5SyXYTV1Dd4k9wr0ZGK88ig at mail.gmail.com...
> Is there a generic binary search routine in a standard library which
>
> a) works for character vectors
> b) runs in O(log(N)) time?
>
> I'm aware of findInterval(x,vec), but it is restricted to numeric vectors.
>
> I'm also aware of various hashing solutions (e.g. new.env(hash=TRUE) and
> fastmatch), but I need the greatest-lower-bound match in my application.
>
> findInterval is also slow for large N=length(vec) because of the O(N)
> checking it does, as Duncan Murdoch has pointed
> out<https://stat.ethz.ch/pipermail/r-help/2008-September/174584.html>:
> though
> its documentation says it runs in O(n * log(N)), it actually runs in O(n *
> log(N) + N), which is quite noticeable for largish N. But that is easy
> enough to work around by writing a variant of findInterval which calls
> find_interv_vec without checking.
>
> -s
>
> PS Yes, binary search is a one-liner in R, but I always prefer to use
> standard, fast native libraries when possible....
>
> binarysearch <- function(val,tab,L,H) {while (H>=L) { M=L+(H-L) %/% 2; if
> (tab[M]>val) H<-M-1 else if (tab[M]<val) L<-M+1 else return(M)};
> return(L-1)}
>
> [[alternative HTML version deleted]]
>
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