spatstat (version 1.23-1)

nnwhich: Nearest neighbour

Description

Finds the nearest neighbour of each point in a point pattern.

Usage

nnwhich(X, ...)
  ## S3 method for class 'ppp':
nnwhich(X, \dots, k=1, method="C")
  ## S3 method for class 'default':
nnwhich(X, Y=NULL, \dots, k=1, method="C")

Arguments

X,Y

Arguments specifying the locations of a set of points. For nnwhich.ppp, the argument X should be a point pattern (object of class "ppp"). For nnwhich.default, typically X and

...

Ignored by nnwhich.ppp and nnwhich.default.

k

Integer, or integer vector. The algorithm will compute the distance to the kth nearest neighbour.

method

String specifying which method of calculation to use. Values are "C" and "interpreted".

Value

• Numeric vector or matrix giving, for each point, the index of its nearest neighbour (or kth nearest neighbour).

  If k = 1 (the default), the return value is a numeric vector v giving the indices of the nearest neighbours (the nearest neighbout of the ith point is the jth point where j = v[i]). If k is a single integer, then the return value is a numeric vector giving the indices of the kth nearest neighbours.

  If k is a vector, then the return value is a matrix m such that m[i,j] is the index of the k[j]th nearest neighbour for the ith data point.

Warnings

A value of NA is returned if there is only one point in the point pattern.

Details

For each point in the given point pattern, this function finds its nearest neighbour (the nearest other point of the pattern). By default it returns a vector giving, for each point, the index of the point's nearest neighbour. If k is specified, the algorithm finds each point's kth nearest neighbour.

The function nnwhich is generic, with method for point patterns (objects of class "ppp") and a default method which are described here, as well as a method for three-dimensional point patterns (objects of class "pp3", described in nnwhich.pp3.

The method nnwhich.ppp expects a single point pattern argument X. The default method expects that X and Y will determine the coordinates of a set of points. Typically X and Y would be numeric vectors of equal length. Alternatively Y may be omitted and X may be a list with two components named x and y, or a matrix or data frame with two columns. The argument k may be a single integer, or an integer vector. If it is a vector, then the $k$th nearest neighbour distances are computed for each value of $k$ specified in the vector.

If there are no points (if x has length zero) a numeric vector of length zero is returned. If there is only one point (if x has length 1), then the nearest neighbour is undefined, and a value of NA is returned. In general if the number of points is less than or equal to k, then a vector of NA's is returned.

The argument method is not normally used. It is retained only for checking the validity of the software. If method = "interpreted" then the distances are computed using interpreted R code only. If method="C" (the default) then C code is used. The C code is faster by two to three orders of magnitude and uses much less memory. To evaluate the distance between a point and its nearest neighbour, use nndist.

To find the nearest neighbours from one point pattern to another point pattern, use nncross.

See Also

nndist, nncross

Examples

data(cells)
   plot(cells)
   m <- nnwhich(cells)
   m2 <- nnwhich(cells, k=2)

   # plot nearest neighbour links
   b <- cells[m]
   arrows(cells$x, cells$y, b$x, b$y, angle=15, length=0.15, col="red")

   # find points which are the neighbour of their neighbour
   self <- (m[m] == seq(m))
   # plot them
   A <- cells[self]
   B <- cells[m[self]]
   plot(cells)
   segments(A$x, A$y, B$x, B$y)