You can't specify it in rthin, which I can understand for the simple

reason that this violates the independence of the sampling. If you put

a fixed limit on your resampling, the chance of being selected depends

on when some point is selected.

You could do it by hacking the object :

rthin.exact <- function(x,n){

l <- x$n

i <- sample(1:l,n,replace=F)

x$n <- n

x$x <- x$x[i]

x$y <- x$y[i]

return(x)

}

data(redwood)

plot(redwood, main="thinning")

# keep 30 points

Y <- rthin.exact(redwood, 30)

points(Y, col="green", cex=1.4)

Cheers

Joris

On Sun, Jun 6, 2010 at 12:26 AM, Tom_R <

[hidden email]> wrote:

>

> Fellow Users,

>

> In Spatstat, if you have a ppp object with, say 100 points, how do you

> create another ppp containing a random selection, N, of the original?

>

> You can tell the function 'rthin' to make a sub-selection of points, i.e.

> "every point is removed with probability 0.5" but you CAN'T set N exactly...

>

> I'm sure this should be easy...

>

> Ta!

>

> Tom

> --

> View this message in context:

http://r.789695.n4.nabble.com/spatstat-sub-selection-tp2244619p2244619.html> Sent from the R help mailing list archive at Nabble.com.

>

> ______________________________________________

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>

https://stat.ethz.ch/mailman/listinfo/r-help> PLEASE do read the posting guide

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--

Ghent University

Faculty of Bioscience Engineering

Department of Applied mathematics, biometrics and process control

tel : +32 9 264 59 87

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