Guide to Prolog Programming	© Roman Barták, 1998
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Boolean Expressions

This chapter extends working with expressions into a different dimension. First, we write a program to evaluate boolean expression which is similar to evaluation of arithmetic expressions. In the second part, we will work with expression in a symbolic manner and we write a program for transformation of a boolean expression into a conjunctive normal form.

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Operator definitons

Before we start to work with boolean (logic) expressions, we define some operators which simplify entry of such expressions.

:-op(720,fy,non).
:-op(730,yfx,and).
:-op(740,yfx,or).

Now, we can write (non a and b) instead of more cumbersome and(non(a),b)

We can also define meta-operations which can be fully transformed into classic operations and, or, non.

:-op(710,yfx,implies).
:-op(710,yfx,equiv).
:-op(740,yfx,xor).

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Evaluating

During evaluation of arithmetic expression we exploit the build-in evaluator is which computes value of numeric expression. Now, we have to define procedures for evaluating and, or, not operations in Prolog.

and_d(false,true,false).
and_d(false,false,false).
and_d(true,false,false).
and_d(true,true,true).
   
or_d(false,true,true).
or_d(false,false,false).
or_d(true,false,true).
or_d(true,true,true).
   
non_d(true,false).
non_d(false,true).

We should also indicate which values can be used in expressions.

logic_const(true).
logic_const(false).

Now, it is easy to write an evaluator for boolean expressions.

eval_b(X,X):-logic_const(X).
eval_b(X and Y,R):-eval_b(X,XV),eval_b(Y,YV),and_d(XV,YV,R).
eval_b(X or Y,R):-eval_b(X,XV),eval_b(Y,YV),or_d(XV,YV,R).
eval_b(non X,R):-eval_b(X,XV),non_d(XV,R).

Evaluation of meta-operations is transformed into evaluation of classic operations in an obvious way.

eval_b(X implies Y,R):-eval_b(Y or non X, R).
eval_b(X equiv Y,R):-eval_b(X implies Y and Y implies X, R).
eval_b(X xor Y,R):-eval_b((X and non Y) or (Y and non X), R).

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Normalizing

In this section, we will write a Prolog program for transformation of boolean expressions into conjunctive normal form. Conjunctive normal form is an expression in the following form:
(a or non b) and c and (b or d or non c).

Note, that we work with arbitrary atoms in boolean expressions now and these atoms are not interpreted, i.e., we do not know their value (true or false).

First, we remove meta-expressions, i.e., implies, equiv & xor which are substituted by or, and & non.

ex2basic(X implies Y, R):-ex2basic(Y or non X,R).
ex2basic(X equiv Y,R):-ex2basic(X implies Y and Y implies X, R).
ex2basic(X xor Y,R):-ex2basic((X and non Y) or (Y and non X), R).
ex2basic(X or Y, XB or YB):-ex2basic(X,XB),ex2basic(Y,YB).
ex2basic(X and Y, XB and YB):-ex2basic(X,XB),ex2basic(Y,YB).
ex2basic(non X, non XB):-ex2basic(X,XB).
ex2basic(X,X):-atom(X).

Second, we move negation non to atomic formulas.

non2basic(non (X and Y),XN or YN):-non2basic(non X,XN),non2basic(non Y,YN).
non2basic(non (X or Y),XN and YN):-non2basic(non X,XN),non2basic(non Y,YN).
non2basic(non non X, XB):-non2basic(X,XB)
non2basic(non X,non X):-atom(X).
non2basic(X and Y,XB and YB):-non2basic(X,XB),non2basic(Y,YB).
non2basic(X or Y,XB or YB):-non2basic(X,XB),non2basic(Y,YB).
non2basic(X,X):-atom(X).

Finally, we can construct a conjunctive normal form.

ex2conj(X and Y,XC and YC):-ex2conj(X,XC),ex2conj(Y,YC).
ex2conj(X or Y, R):-ex2conj(X,XC),ex2conj(Y,YC),join_disj(XC,YC,R).
ex2conj(non X,non X).
ex2conj(X,X):-atom(X).
   
join_disj(X and Y,Z,XZ and YZ):-join_disj(X,Z,XZ),join_disj(Y,Z,YZ).
join_disj(X,Y and Z,XY and XZ):-X\=(_ and _),join_disj(X,Y,XY),join_disj(X,Z,XZ).
join_disj(X,Y,X or Y):-X\=(_ and _),Y\=(_ and _).

Now, we join above three procedures into compact form which transforms arbitrary expression into its conjuctive normal form. We call this process normalization.

normalize(Ex,Norm):-
   ex2basic(Ex,Bas),
   non2basic(Bas,NonBas),
   ex2conj(NonBas,Norm).

Try to optimize the resulting conjuctive normal form by removing disjunctions containing literal and its negation (e.g., a or non a, which is known to be true). Write similar procedure(s) for transformation into disjunctive normal form.

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See also:

Arithmetic Expressions

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