Subsetting Sparse Symmetric Matrices

Description

This function conveniently returns a subset (submatrix) of a sparse symmetric (distance) matrix.

Usage

## S3 method for class 'dist':
subset(x, subset, ...)
## S3 method for class 'dist':
x[[subset, ...]]

x[[i]]

Arguments

Details

For consistency, subsetting a symmetric matrix is constrained to a single index (vector). However, non-unique indexing is allowed. In this case zero is returned for diagonal elements.

If x has a Labels attribute a character vector can be used for subsetting.

Note that an error is raised if subset contains NA, or a non-finite value.

Value

An object of class dist.

Warning

This is experimental code which may change in future releases.

Author(s)

Christian Buchta

See Also

dist for details.

Examples

x <- as.dist(matrix(1:36, ncol=6))
subset(x, c(2,4,6))
subset(x, c(4,2,6))               # same set but different order
attr(x, "Labels") <- letters[1:6] # hack!
subset(x, c(2,4,6))
subset(x, letters[c(2,4,6)])
subset(x, c(2,2,4,6))             # contains one zero 
# this is more user friendly
x[[c(2,4,6)]]
x[[c("b","d","e")]]
## finally, don't use NA, etc.
z <- try(x[[c(2,NA,6)]], silent=TRUE)
z