CODEWITHSUNDEEP
prologtransitive-closureknowledge-management
Zhang Xinye
Zhang Xinye

Reputation: 73

In Prolog, how to make comparison when order is defined in a chain of predicates?

Having the following definition:

biggerThan(a,b).
biggerThan(b,c).
biggerThan(c,d).

How to define a rule is_bigger(X,Y), such that is_bigger(a,c) and is_bigger(a,d) will return true.

Besides, I am very new to Prolog. Did the title properly addressed the problem, if not, how should I say is?

Upvotes: 0

Views: 149

Answers (1)

Andrey Tyukin
Andrey Tyukin

Reputation: 44918

Simply define is_bigger as the transitive closure of the biggerThan relation:

biggerThan(a,b).
biggerThan(b,c).
biggerThan(c,d).

is_bigger(X, Y) :- biggerThan(X, Y).
is_bigger(X, Y) :- biggerThan(X, Z), is_bigger(Z, Y).

The transitive closure of a relation R happens to be the smallest relation X such that R is contained in X (this is the first clause of the definition), and such that X = R o X (this is the second part). So, the above is basically one possible definition written out.

Upvotes: 1

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