Topological Sort

Description

Find One Total Order Consistent with Partial Order or With Directed Acyclic Graph

Usage

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tsort(from, to, domain, strict = TRUE)

Arguments

from

an atomic vector

to

an atomic vector of the same mode and length as from

domain

an atomic vector of the same mode as from containing all the elements of from and to. If missing, union(from, to) is used. The domain of the relation or the nodes of the graph.

strict

logical. If TRUE then from[i] == to[i] is an error. Otherwise, such edges of the graph are ignored.

Details

Pairs (from[i], to[i]) can be though of either as elements of a relation on a set or as edges in a directed graph. This function finds one total order on the domain (nodes of the graph) that is consistent with the relation (graph) if one exists (that is if the graph is directed).

Value

A vector that is a reordering of domain so that every element of from appears in the value before the corresponding element of to.

Throws an error if there is no consistent total order (the graph has a cycle).

Examples

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from <-   LETTERS[c(1, 1, 1, 1, 2, 2, 6)]
to <- LETTERS[c(2, 3, 4, 5, 3, 5, 7)]
from
to
tsort(from, to)

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