Math 156
Discrete Mathematics for Computer Science
Fall 1999

Toy Prolog Examples

This page contains several example programs:

Grandparents and Siblings

Aunts and Uncles -- First Attempt

Notice that this examples fails to get uncles and aunts by marriage. We'll add them in in the next example.

Aunts and Uncles -- Second Attempt

childOf(jay,john,jennie).
childOf(ron,john,jennie).
childOf(russ,john,jennie).
childOf(roger,john,jennie).
 
childOf(randy,jay,bev).
childOf(julie,jay,bev).
childOf(brad,jay,bev).
childOf(mark,jay,bev).
childOf(jen,jay,bev).

childOf(kevin,ron,peg).
childOf(jenifer,ron,peg).

childOf(michelle,russ,kathy).
childOf(brian,russ,kathy).

childOf(justin,roger,laura).
childOf(brent,roger,laura).
childOf(chelsea,roger,laura).

childOf(andrew,randy,penny).
childOf(anika,randy,penny).
childOf(jason,randy,penny).

childOf(leah,mike,julie).
childOf(braden,brad,laura2).

married(mark,chriss).
married(ron,dianna).

equal(X,X).
married(A,B) <= childOf(Child,A,B).
married(A,B) <= childOf(Child,B,A).

siblings(X,Y) <= childOf(X,Par1,Par2) and childOf(Y,Par1,Par2) and not equal(X,Y).

childOf(C,P) <= childOf(C,P,X).
childOf(C,P) <= childOf(C,X,P).

dUncOf(U,N) <= childOf(N,Parent) and siblings(U,Parent).
mUncOf(U,N) <= dUncOf(X,N) and married(U,X).
uncOf(U,N) <= dUncOf(U,N).
uncOf(U,N) <= mUncOf(U,N).

? uncOf(X,andrew).

Oops. This still isn't correct. chriss should be an aunt of andrew since she is married to mark. The problem is that we have not made married symetric.

Aunts and Uncles -- Third Attempt

childOf(kevin,ron,peg).
childOf(jenifer,ron,peg).

childOf(michelle,russ,kathy).
childOf(brian,russ,kathy).

childOf(justin,roger,laura).
childOf(brent,roger,laura).
childOf(chelsea,roger,laura).

childOf(andrew,randy,penny).
childOf(anika,randy,penny).
childOf(jason,randy,penny).

childOf(leah,mike,julie).
childOf(braden,brad,laura2).

married(mark,chriss).
married(ron,dianna).

equal(X,X).
married(A,B) <= childOf(Child,A,B).
{married(A,B) <= childOf(Child,B,A).} {don't need this now.}
married(A,B) <= married(B,A).

siblings(X,Y) <= childOf(X,Par1,Par2) and childOf(Y,Par1,Par2) and not equal(X,Y).

childOf(C,P) <= childOf(C,P,X).
childOf(C,P) <= childOf(C,X,P).

dUncOf(U,N) <= childOf(N,Parent) and siblings(U,Parent).
mUncOf(U,N) <= dUncOf(X,N) and married(X,U).
uncOf(U,N) <= dUncOf(U,N).
uncOf(U,N) <= mUncOf(U,N).

? uncOf(X,andrew).

Now what?!? We have an infinite loop! Why? The program goes back and forth figuring out that, for example, mark and chriss are married over and over. (

married(mirk,chriss) => married(chriss,mark) => married(mark, chriss)

, etc.) Actually, it gets stuck even sooner when it is figuring out about mike and julie. One fix is given in the next attempt.

Aunts and Uncles -- Fourth Attempt

childOf(kevin,ron,peg).
childOf(jenifer,ron,peg).

childOf(michelle,russ,kathy).
childOf(brian,russ,kathy).

childOf(justin,roger,laura).
childOf(brent,roger,laura).
childOf(chelsea,roger,laura).

childOf(andrew,randy,penny).
childOf(anika,randy,penny).
childOf(jason,randy,penny).

childOf(leah,mike,julie).
childOf(braden,brad,laura2).

husbandwife(mark,chriss).
husbandwife(ron,dianna).

equal(X,X).
husbandwife(A,B) <= childOf(Child,A,B).
wifehusband(A,B) <= childOf(Child,B,A).
married(A,B) <= husbandwife(A,B).
married(A,B) <= wifehusband(A,B).

siblings(X,Y) <= childOf(X,Par1,Par2) and childOf(Y,Par1,Par2) and not equal(X,Y).

childOf(C,P) <= childOf(C,P,X).
childOf(C,P) <= childOf(C,X,P).

dUncOf(U,N) <= childOf(N,Parent) and siblings(U,Parent).
mUncOf(U,N) <= dUncOf(X,N) and married(X,U).
uncOf(U,N) <= dUncOf(U,N).
uncOf(U,N) <= mUncOf(U,N).

? uncOf(X,andrew).

Is she a witch?

The example above is based on Scene 5 of Monty Python's Search for the Holy Grail. If you are interested in the Monty Python sketch, here are links to the text and audio

Last modified: Monday, 30-Apr-2001 03:11:49 EDT

Math 156 Discrete Mathematics for Computer Science Fall 1999

Toy Prolog Examples

Grandparents and Siblings

Aunts and Uncles -- First Attempt

Aunts and Uncles -- Second Attempt

Aunts and Uncles -- Third Attempt

Aunts and Uncles -- Fourth Attempt

Is she a witch?

Math 156
Discrete Mathematics for Computer Science
Fall 1999