overview prolog

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An Overview of Prolog

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What is Prolog Prolog is a powerful language for AI and non-

numerical programming Stands for programming in logic Centered around a small set of basic

mechanisms, including: Pattern matching Tree-based data structuring Automatic back tracking

Well suited for problems that involves structured objects and relations between them.

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Chapter One: Introduction to Prolog1. Defining relations by facts2. Defining relations by rules3. Recursive rules4. How Prolog answers questions5. Declarative and procedural meaning of

programs

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Chapter one:

1. Defining relations by facts2. Defining relations by rules

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1.1 Defining relations by factsExampleThe fact that tom is a parent of bobparent(tom, bob).

the name of a relation argument1 argument2

Fact clause

tom bob annpat jimliz

pam

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1.1 Defining relations by facts The family tree is defined by the following

Prolog program:parent(pam, bob)parent(tom, bob)parent(tom, liz)parent(bob, ann)parent(bob, pat)parent(pat, jim)This program consists of six clauses.

Each clause declares one fact about parent relation (particular instance of the parent relation) * a relation is the set of all its instances


pam

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1.1 Defining relations by factsWhen the program communicated to Prolog system,

Prolog can be posed some questions about the parent relation:

Is bob a parent of pat??- parent(bob, pat)Prolog answer: True

?- parent(liz, pat)Prolog answer: No solutions

?- parent(tom, pat)

Prolog answer: No solutions


pam

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1.1 Defining relations by factsWho is liz’s parent??- parent(X, liz)Prolog answer: X = tom

Who are bob’s children??- parent(bob, X)Prolog answer: X = ann.X = pat.


pam

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1.1 Defining relations by factsFind X and Y such that X is a parent of Y.?- parent (X, Y).Prolog answers: X = pam.Y = bob.

X = tom.Y = bob.

X = tom.Y = liz.

X = bob.Y = ann.

X = bob.Y = pat.

X = pat.Y = jim.


pam

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1.1 Defining relations by factsWho is a grandparent of jim?Who is the parent of jim? Assume that this is YWho is the parent of Y? Assume that this is X

?- parent(Y, jim), parent (X, Y)Prolog answer: X = bob Y = pat

• Changing the order of the requirements will affect the result.?- parent(X, jim), parent (Y, X)

Prolog answer: X = pat. Y = bob.

?- parent(X, jim), parent (X, Y) Prolog answer: X = pat. Y = jim.


pam

√

xx

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1.1 Defining relations by factsDo ann and pat have a common parent?Who is a parent X of ann?Is X a parent of pat?

?- parent (X, ann), parent(X, pat)Prolog answer: X = bob


pam

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1.2 Defining relations by rules We can add the information on the sex of the

people that occur in the parent relation:female(pam).male(tom).male(bob).female(liz).female(pat).female(ann).male(jim).male and female are unary relations,

whereas parent is a binary relation.

Gender(pam, female).Gender(tom, male).Gender(bob, male).


pam

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1.2 Defining relations by rulesLet us introduce the offspring relation (inverse

of parent)offspring(liz, tom).offspring(bob, tom)....

The logical statement is:For all X and Y, Y is an offspring of X ifX is a parent of Yoffspring(Y, X) :- parent(X, Y). Rule clause


pam

Fact caluses

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1.2 Defining relations by rulesThere is an important difference between facts

and rules: Facts is something that is always,

unconditionally, trueparent(tom, liz).

Rules specify things that are true if some condition is satisfied. Rules have: Condition part (right-hand side) = body Conclusion part (left-hand side) = head

offspring(Y, X) :- parent(X, Y). head body

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1.2 Defining relations by rulesQuestions Is liz an offspring of tom??- offspring(liz, tom).Prolog answer: TrueHow Prolog answer this question using the rule:Offspring(Y, X) :- parent(X, Y).

Prolog answer: True

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1.2 Defining relations by rules How to express:

Mother relation Grandparent relation Sister relation

in Prolog?

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Important Points

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