elements {relations}

Elements of Relation Domains

Description

Obtain elements of endorelation domains which have certain properties.

Usage

relation_elements(x, which, ...)

Arguments

x

an endorelation.

which

a character string specifying the property to be tested for. Currently, one of "minimal", "first", "last", or "maximal", or a unique abbreviation thereof.

...

additional arguments to be employed in the property tests.

Details

Let R be an endorelation with domain (X, X) and consider elements x and y of X. We say that x is

minimal:

there is no y \ne x with y R x.

a first element:

x R y for all y \ne x.

a last element:

y R x for all y \ne x.

maximal:

there is no y \ne x with x R y.

When computing the tests for the above properties, an additional na.rm argument can be given to control the handling of missing incidences. By default, these are treated as false, to the effect that they invalidate “for all” tests (corresponding to na.rm = FALSE) and pass the “there is no” tests (corresponding to na.rm = TRUE).

Value

A set with the elements having the specified property.