[R] General binary search?
William Dunlap
wdunlap at tibco.com
Mon Apr 4 22:50:28 CEST 2011
> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of Stavros Macrakis
> Sent: Monday, April 04, 2011 1:15 PM
> To: r-help
> Subject: [R] General binary search?
>
> 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.
xtfrm(x) will convert a character (or other) vector to
a numeric vector with the same ordering. findInterval
can work on that. E.g.,
> f0 <- function(x, vec) {
tmp <- xtfrm(c(x, vec))
findInterval(tmp[seq_along(x)], tmp[-seq_along(x)])
}
> f0(c("Baby", "Aunt", "Dog"), LETTERS)
[1] 2 1 4
I've never looked at its speed.
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
>
> 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)}
>
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>
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