# Prolog programming Introduction to Prolog (part4) CS 370 – CS461.

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Prolog programming Introduction to Prolog (part4) CS 370 – CS461

How to solve this problem? Consider the following facts: parent(pam,bob). parent(tom,bob). parent(tom,liz). parent(bob,ann). parent(bob,pat). parent(pat,jim). If we want to describe a new relation called “ predecessor”, which checks if a specific node is a predecessor of another node or not. For example: ?- predecessor(tom, pat). How can this relation be described? pam tom bobliz annpat jim

Defining relations by Recursive Rule. To solve the previous predecessor(tom, pat) relation problem we have to defined a recursive rule. This relation will be defined in terms of the parent relation. The whole definition can be expressed with two rules: 1.First rule will define the direct predecessor. 2.Second rule will be the indirect predecessors.

Defining relations by Recursive Rule. 1.Direct Rule  For all X and Z X is a predecessor of Z if X is a parent of Z.  The rule will be: predecessor( X, Z ) :- parent( X, Z ). x z Predecessor parent

Defining relations by Recursive Rule. 2. Indirect Rule Y1 Y2 z Predecessor parent x Y1 z Y2 Predecessor parent Predecessor

Defining relations by Recursive Rule. 2. Indirect Rule  For all X and Z there is a Y such that: X is a predecessor of Z if X is a parent of Y and Y is a predecessor of Z.  The rule will be: predecessor( X, Z ) :- parent( X, Y), predecessor(Y,Z).

Defining relations by Recursive Rule. Thus,we have constructed a complete program for the predecessor relation, which consists of two rules: one for direct predecessors and one for indirect predecessors. Both rules must be rewritten together as : predecessor( X, Z ) :- parent( X, Z ). predecessor( X, Z ) :- parent( X, Y ), predecessor( Y, Z ).

Class exercise(1) Write a rule to describe the successor relation?

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