[R] selecting clusters of points

[Roger Bivand]

On Fri, 25 Jan 2002, Tony Long wrote:

> 	Are there any functions out there for selecting all the 
> points in a region of a plot.  I envision something like the 
> identify() function except one could circle a cloud of points (and 
> perhaps a vector would be returned of the same length as the points 
> plotted indicating logical membership in the circled cloud).  Perhaps 
> someone has done something with the locator() function that would 
> allow this (or can suggest a way I could do this).   Thanks.  Tony
> 
At least one possibility is:

> library(splancs)
> poly <- cbind(x=c(0,0,1,1), y=c(0,1,1,0))
> data <- csr(poly, 50)
> plot(data)
> subset <- getpoly()
 -- here interactively choose your polygon --
> inout(data, subset)
 -- outputs logical vector for points within subset polygon --

If you need to resolve for points precisely on the polygon boundaries,
pip() may be more help. The subset polygon could be constructed
non-interactively too, but needs to be a polygon.

Roger

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand at nhh.no
and: Department of Geography and Regional Development, University of
Gdansk, al. Mar. J. Pilsudskiego 46, PL-81 378 Gdynia, Poland.

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