pairdist.default.Rd
\name{pairdist.default}
\alias{pairdist.default}
\title{Pairwise distances}
\description{
Computes the matrix of distances between all pairs of points
in a set of points
}
\usage{
\method{pairdist}{default}(X, Y=NULL, \dots, period=NULL, method="C", squared=FALSE)
}
\arguments{
\item{X,Y}{
Arguments specifying the coordinates of a set of points.
Typically \code{X} and \code{Y} would be
numeric vectors of equal length.
Alternatively \code{Y} may be omitted and \code{X} may be
a list with two components \code{x} and \code{y},
or a matrix with two columns.
}
\item{\dots}{
Ignored.
}
\item{period}{
Optional. Dimensions for periodic edge correction.
}
\item{method}{
String specifying which method of calculation to use.
Values are \code{"C"} and \code{"interpreted"}.
Usually not specified.
}
\item{squared}{
Logical. If \code{squared=TRUE}, the squared distances are
returned instead (this computation is faster).
}
}
\value{
A square matrix whose \code{[i,j]} entry is the distance
between the points numbered \code{i} and \code{j}.
}
\details{
Given the coordinates of a set of points,
this function computes the Euclidean distances between all pairs of
points, and returns the matrix of distances.
It is a method for the generic function \code{pairdist}.
The arguments \code{X} and \code{Y} must determine
the coordinates of a set of points. Typically \code{X} and
\code{Y} would be numeric vectors of equal length. Alternatively
\code{Y} may be omitted and \code{X} may be a list with two components
named \code{x} and \code{y}, or a matrix or data frame with two columns.
Alternatively if \code{period} is given,
then the distances will be computed in the `periodic'
sense (also known as `torus' distance).
The points will be treated as if they are in a rectangle
of width \code{period[1]} and height \code{period[2]}.
Opposite edges of the rectangle are regarded as equivalent.
If \code{squared=TRUE} then the \emph{squared} Euclidean distances
\eqn{d^2} are returned, instead of the Euclidean distances \eqn{d}.
The squared distances are faster to calculate, and are sufficient for
many purposes (such as finding the nearest neighbour of a point).
The argument \code{method} is not normally used. It is
retained only for checking the validity of the software.
If \code{method = "interpreted"} then the distances are
computed using interpreted R code only. If \code{method="C"}
(the default) then C code is used. The C code is somewhat faster.
}
\seealso{
\code{\link{crossdist}},
\code{\link{nndist}},
\code{\link{Kest}}
}
\examples{
x <- runif(100)
y <- runif(100)
d <- pairdist(x, y)
d <- pairdist(cbind(x,y))
d <- pairdist(x, y, period=c(1,1))
d <- pairdist(x, y, squared=TRUE)
}
\author{Pavel Grabarnik
\email{pavel.grabar@issp.serpukhov.su}
and
\adrian
}
\keyword{spatial}
\keyword{math}