[R] own function: computing time

[William Dunlap]

Your original method would be the following function
f <- function (x, y) 
{
    xy <- cbind(x, y)
    outside <- function(z) {
        !any(x > z[1] & y > z[2])
    }
    j <- apply(xy, 1, outside)
    which(j)
}

and the following one quickly computes the same thing as the above
as long as there are no repeated points (if there are repeated
points it chooses one of them).

f1 <- function (x, y) 
{
    o <- order(x, decreasing = TRUE)
    yo <- y[o]
    j <- logical(length(y))
    j[o] <- yo == cummax(yo)
    which(j)
}

Think of the problem as finding the "ladder points" (Feller's term)
of a sequence of points, the places where the sequence reaches
a new high point.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of William Dunlap
> Sent: Wednesday, October 10, 2012 9:52 AM
> To: tonja.krueger at web.de; r-help at r-project.org
> Subject: Re: [R] own function: computing time
> 
> No, the desired points are not a subset of the convex hull.
> E.g., x=c(0,1:5), y=c(0,1/(1:5)).
> 
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com
> 
> 
> > -----Original Message-----
> > From: William Dunlap
> > Sent: Wednesday, October 10, 2012 9:46 AM
> > To: 'tonja.krueger at web.de'; r-help at r-project.org
> > Subject: RE: [R] own function: computing time
> >
> > Are the points you are looking for (those data points with no other data
> > points above or to the right of them) a subset of the convex hull of the
> > data points?  If so, chull(x,y) can quickly give you the points on the convex
> > hull (typically a fairly small number) and you can look through them for
> > the ones you want.
> >
> > Bill Dunlap
> > Spotfire, TIBCO Software
> > wdunlap tibco.com
> >
> >
> > > -----Original Message-----
> > > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
> Behalf
> > > Of tonja.krueger at web.de
> > > Sent: Wednesday, October 10, 2012 3:16 AM
> > > To: r-help at r-project.org
> > > Subject: [R] own function: computing time
> > >
> > > Hi all,
> > >
> > > I wrote a function that actually does what I want it to do, but it tends to be very slow
> > for
> > > large amount of data. On my computer it takes 5.37 seconds for 16000 data points
> and
> > > 21.95 seconds for 32000 data points. As my real data consists of 18000000 data
> points
> > it
> > > would take ages to use the function as it is now.
> > > Could someone help me to speed up the calculation?
> > >
> > > Thank you, Tonja
> > >
> > > system.time({
> > > x <- runif(32000)
> > > y <- runif(32000)
> > >
> > > xy <- cbind(x,y)
> > >
> > > outer <- function(z){
> > > !any(x > z[1] & y > z[2])}
> > > j <- apply(xy,1, outer)
> > >
> > > plot(x,y)
> > > points(x[j],y[j],col="green")
> > >
> > > })
> > >
> > > ______________________________________________
> > > R-help at r-project.org mailing list
> > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained, reproducible code.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.