CnfFormula: CNF Formulas

In mlr3pipelines: Preprocessing Operators and Pipelines for 'mlr3'

CNF Formulas

Description

A CnfFormula is a conjunction of CnfClause objects. It represents a statement that is true if all of the clauses are true. These are for example of the form

  (X %among% "a" | Y %among% "d") & Z %among% "g"

CnfFormula objects can be constructed explicitly, using the CnfFormula() constructor, or implicitly, by using the & operator on CnfAtoms, CnfClauses, or other CnfFormula objects.

To get individual clauses from a formula, [[ should not be used; instead, use as.list(). Note that the simplified form of a formula containing a tautology is the empty list.

Upon construction, the CnfFormula is simplified by using various heuristics. This includes unit propagation, subsumption elimination, self/hidden subsumption elimination, hidden tautology elimination, and resolution subsumption elimination (see examples). Note that the order of clauses in a formula is not preserved.

Using CnfFormula() on lists that contain other CnfFormula objects will create a formula that is the conjunction of all clauses in all formulas. This may be somewhat more efficient than applying & many times in a row.

If a CnfFormula contains no clauses, or only TRUE clauses, it evaluates to TRUE. If it contains at least one clause that is, by itself, always false, the formula evaluates to FALSE. Not all contradictions between clauses are recognized, however. These values can be converted to, and from, logical(1) values using as.logical() and as.CnfFormula().

CnfFormula objects can be negated using the ! operator. Beware that this may lead to an exponential blow-up in the number of clauses.

This is part of the CNF representation tooling, which is currently considered experimental; it is for internal use.

Usage

CnfFormula(clauses)

as.CnfFormula(x)

Arguments

clauses	(list of CnfClause) A list of CnfClause objects. The formula represents the conjunction of these clauses.
x	(any) The object to be coerced to a CnfFormula by as.CnfFormula. Only logical(1), CnfAtom, CnfClause, and CnfFormula itself are currently supported.

Value

A new CnfFormula object.

See Also

Other CNF representation objects: CnfAtom(), CnfClause(), CnfSymbol(), CnfUniverse()

Examples

u = CnfUniverse()
X = CnfSymbol(u, "X", c("a", "b", "c"))
Y = CnfSymbol(u, "Y", c("d", "e", "f"))
Z = CnfSymbol(u, "Z", c("g", "h", "i"))

frm = (X %among% c("a", "b") | Y %among% c("d", "e")) &
  Z %among% c("g", "h")
frm

# retrieve individual clauses
as.list(frm)

# Negation of a formula
# Note the parentheses, otherwise `!` would be applied to the first clause only.
!((X %among% c("a", "b") | Y %among% c("d", "e")) &
   Z %among% c("g", "h"))

## unit propagation
# The second clause can not be satisfied when X is "b", so "b" can be
# removed from the possibilities in the first clause.
(X %among% c("a", "b") | Y %among% c("d", "e")) &
  X %among% c("a", "c")

## subsumption elimination
# The first clause is a subset of the second clause; whenever the
# first clause is satisfied, the second clause is satisfied as well, so the
# second clause can be removed.
(X %among% "a" | Y %among% c("d", "e")) &
  (X %among% c("a", "b") | Y %among% c("d", "e") | Z %among% "g")

## self subsumption elimination
# If the first clause is satisfied but X is not "a", then Y must be "e".
# The `Y %among% "d"` part of the first clause can therefore be removed.
(X %among% c("a", "b") | Y %among% "d") &
  (X %among% "a" | Y %among% "e")

## resolution subsumption elimination
# The first two statements can only be satisfied if Y is either "d" or "e",
# since when X is "a" then Y must be "e", and when X is "b" then Y must be "d".
# The third statement is therefore implied by the first two, and can be
# removed.
(X %among% "a" | Y %among% "d") &
  (X %among% "b" | Y %among% "e") &
  (Y %among% c("d", "e"))

## hidden tautology elimination / hidden subsumption elimination
# When considering the first two clauses only, adding another atom
# `Z %among% "i"` to the first clause would not change the formula, since
# whenever Z is "i", the second clause would need to be satisfied in a way
# that would also satisfy the first clause, making this atom redundant
# ("hidden literal addition"). Considering the pairs of clause 1 and 3, and
# clauses 1 and 4, one could likewise add `Z %among% "g"` and
#' `Z %among% "h"`, respectively. This would reveal the first clausee to be
# a "hidden" tautology: it is equivalent to a clause containing the
# atom `Z %among% c("g", "h", "i")` == TRUE.
# Alternatively, one could perform "hidden" resolution subsumption using
# clause 4 after having added the atom `Z %among% c("g", "i")` to the first
# clause by using clauses 2 and 3.
(X %among% c("a", "b") | Y %among% c("d", "e")) &
  (X %among% "a" | Z %among% c("g", "h")) &
  (X %among% "b" | Z %among% c("h", "i")) &
  (Y %among% c("d", "e") | Z %among% c("g", "i"))

## Simple contradictions are recognized:
(X %among% "a") & (X %among% "b")
# Tautologies are preserved
(X %among% c("a", "b", "c")) & (Y %among% c("d", "e", "f"))

# But not all contradictions are recognized.
# Builtin heuristic CnfFormula preprocessing is not a SAT solver.
contradiction = (X %among% "a" | Y %among% "d") &
  (X %among% "b" | Y %among% "e") &
  (X %among% "c" | Y %among% "f")
contradiction

# Negation of a contradiction results in a tautology, which is recognized
# and simplified to TRUE. However, note that this operation (1) generally has
# exponential complexity in the number of terms and (2) is currently also not
# particularly well optimized
!contradiction

mlr3pipelines documentation built on Sept. 30, 2024, 9:37 a.m.