5.226. lex_chain_lesseq
DESCRIPTION | LINKS | GRAPH |
- Origin
- Constraint
- Usual name
- Type
- Argument
- Restrictions
- Purpose
For each pair of consecutive vectors and of the collection we have that is lexicographically less than or equal to . Given two vectors, and of components, and , is lexicographically less than or equal to if and only if or or and is lexicographically less than or equal to .
- Example
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The constraint holds since:
The first vector of the collection is lexicographically less than or equal to the second vector of the collection.
The second vector of the collection is lexicographically less than or equal to the third vector of the collection.
- Typical
- Arg. properties
Contractible wrt. .
Suffix-contractible wrt. (remove items from same position).
- Usage
This constraint was motivated for breaking symmetry: more precisely when one wants to lexicographically order the consecutive columns of a matrix of decision variables. A further motivation is that using a set of lexicographic ordering constraints between two vectors does usually not allows to come up with a complete pruning.
- Algorithm
A filtering algorithm achieving arc-consistency for a chain of lexicographical ordering constraints is presented inΒ [BeldiceanuCarlsson02c].
Six different ways of integrating a chain of lexicographical ordering constraints within non-overlapping constraints like or and within their corresponding necessary condition like the constraint are shown inΒ [AgrenCarlssonBeldiceanuSbihiTruchetZampelli09].
- Systems
lexChainEq in Choco, lex_chain in SICStus.
- See also
common keyword: Β (lexicographic order), Β (symmetry, lexicographic ordering on the origins of , , ), , , , Β (lexicographic order).
implied by: Β (columns lex ordering imposed by constraint removed), Β (non-strict order implied by strict order),
part of system of constraints: .
related: , Β (lexicographic ordering on the origins of , , ).
- Keywords
characteristic of a constraint: vector.
constraint type: system of constraints, decomposition, order constraint.
- Arc input(s)
- Arc generator
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- Arc arity
- Arc constraint(s)
- Graph property(ies)
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- Graph model
PartsΒ (A) andΒ (B) of FigureΒ 5.226.1 respectively show the initial and final graph associated with the Example slot. Since we use the graph property, the arcs of the final graph are stressed in bold. The constraint holds since all the arc constraints of the initial graph are satisfied.
Figure 5.226.1. Initial and final graph of the constraint
(a) (b) - Signature
Since we use the arc generator on the collection the number of arcs of the initial graph is equal to . For this reason we can rewrite to and simplify to .