| nearestdist {spam} | R Documentation |
This function computes and returns specific elements of distance matrix computed by using the specified distance measure.
nearest.dist( x, y=NULL, method = "euclidean",
eps = .Spam$eps, delta = 1,
diag = FALSE, upper = FALSE,
p=2, miles=TRUE, R=NULL)
x |
Matrix of first set of locations where each row gives the coordinates of a particular point. See also Details. |
y |
Matrix of second set of locations where each row gives the
coordinates of a particular point. If this is missing x is
used. See also Details. |
method |
the distance measure to be used. This must be one of
"euclidean", "maximum", "minkowski" or
"greatcircle". Any unambiguous substring can be
given. |
eps |
distances smaller than this number are considered zero. |
delta |
only distances smaller than delta are recorded. |
diag |
Should the diagonal be included? See details. |
upper |
Should the entire matrix (NULL) or only the upper-triagonal (TRUE)
or lower-triagonal (FALSE) values be calculated. |
p |
The power of the Minkowski distance. |
miles |
For great circle distance: If true distances are in statute miles if false distances in kilometers. |
R |
For great circle distance: Radius to use for sphere to find spherical distances. If NULL
the radius is either in miles or kilometers depending on the
values of the miles argument. If R=1 then distances are of
course in radians. |
For great circle distance, the by 2 matrices x and y contain
the degrees longitudes in the first and the degrees latitudes in the second column.
eps and delta are in degrees. The distance is
in single precision (I am still not sure where I loose the double in
the Fortran code) and if calculating the entire matrix
upper=NULL (instead of adding its transpose) it may not
pass the symmetry checks, for example.
The argument dist=TRUE determines if diagonal elements will also be
included if smaller than eps. This is useful when calculating
covariance matrices based on a distance matrix. The default values of
dist=FALSE and upper=FALSE are borrowed from dist.
x and y can be any object with an existing
as.matrix method.
A quick scan revieled distance functions in at least 7 packages. The
argument names should be as general as possible and be coherend with
many (but not all) available distance functions.
The Fortran code is based on a idea of Doug Nychka.
A spam object containing the distances spanned by eps and delta.
Reinhard Furrer
# Note that upper=T and using t(X)+X is quicker than upper=NULL;
# upper=T marginally slower than upper=F.
# To compare nearest.dist with dist, use diag=FALSE, upper=TRUE
nx <- 4
x <- expand.grid(as.double(1:nx),as.double(1:nx))
sum( (nearest.dist( x, delta=nx*2, diag=FALSE, upper=TRUE)@entries-
c(dist(x)))^2)
# Create nearest neighbor structures:
par(mfcol=c(1,2))
x <- expand.grid(1:nx,1:(2*nx))
display( nearest.dist( x, delta=1))
x <- expand.grid(1:(2*nx),1:nx)
display( nearest.dist( x, delta=1))