[R] boundary check
Michael Bedward
michael.bedward at gmail.com
Fri Sep 24 12:44:18 CEST 2010
Hello,
If an N-dimensional convex hull fits your idea of a "smallest ball"
then you could try the convhulln function in the geometry package.
For testing if a new point is inside a previously derived hull, one
brute force approach is to rbind the new point to your data, generate
a new hull and see if it is the same as the previous one.
I've only used convhulln in low dimensions so I don't know how
efficient it is when N is large.
Hope this helps.
Michael
On 24 September 2010 19:44, Feng Li <feng.li at stat.su.se> wrote:
> Dear R,
>
> I have a covariates matrix with 10 observations, e.g.
>
>> X <- matrix(rnorm(50), 10, 5)
>> X
> [,1] [,2] [,3] [,4] [,5]
> [1,] 0.24857135 0.30880745 -1.44118657 1.10229027 1.0526010
> [2,] 1.24316806 0.36275370 -0.40096866 -0.24387888 -1.5324384
> [3,] -0.33504014 0.42996246 0.03902479 -0.84778875 -2.4754644
> [4,] 0.06710229 1.01950917 -0.09325091 -0.03222811 0.4127816
> [5,] -0.13619141 1.33143821 -0.79958805 2.08274102 0.6901768
> [6,] -0.45060357 0.19348831 -1.23793647 -0.72440163 0.5057326
> [7,] -1.20740516 0.20231086 1.15584485 0.81777770 -1.2719855
> [8,] -1.81166284 -0.07913113 -0.91080581 -0.34774436 0.9552182
> [9,] 0.19131383 0.14980569 -0.37458224 -0.09371273 -1.7667203
> [10,] -0.85159276 -0.66679528 1.63019340 0.56920196 -2.4049600
>
> And I define a boundary of X: The smallest "ball" that nests all the
> observations of X. I wish to check if a particular point x_i
>
>> x_i <- matrix(rnorm(5), 1, 5)
>> x_i
> [,1] [,2] [,3] [,4] [,5]
> [1,] -0.1525543 0.4606419 -0.1011011 -1.557225 -1.035694
>
> is inside the boundary of X or not. I know it's easy to do it with 1-D or
> 2-D, but I don't knot how to manage it when the dimension is large.
>
> Can someone give a hint? Thanks in advance!
>
>
> Feng
>
> --
> Feng Li
> Department of Statistics
> Stockholm University
> 106 91 Stockholm, Sweden
> http://feng.li/
>
> [[alternative HTML version deleted]]
>
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