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### CS621: Artificial Intelligence

Pushpak BhattacharyyaCSE Dept., IIT Bombay

Lecture 35–Himalayan Club example; introducing Prolog

Himalayan Club example

- Introduction through an example (Zohar Manna, 1974):
- Problem: A, B and C belong to the Himalayan club. Every member in the club is either a mountain climber or a skier or both. A likes whatever B dislikes and dislikes whatever B likes. A likes rain and snow. No mountain climber likes rain. Every skier likes snow. Is there a member who is a mountain climber and not a skier?
- Given knowledge has:
- Facts
- Rules

Example contd.

- Let mc denote mountain climber and sk denotes skier. Knowledge representation in the given problem is as follows:
- member(A)
- member(B)
- member(C)
- ∀x[member(x) → (mc(x) ∨ sk(x))]
- ∀x[mc(x) → ~like(x,rain)]
- ∀x[sk(x) → like(x, snow)]
- ∀x[like(B, x) → ~like(A, x)]
- ∀x[~like(B, x) → like(A, x)]
- like(A, rain)
- like(A, snow)
- Question: ∃x[member(x) ∧ mc(x) ∧ ~sk(x)]
- We have to infer the 11th expression from the given 10.
- Done through Resolution Refutation.

Club example: Inferencing

- member(A)
- member(B)
- member(C)
- Can be written as

Assignment

- Prove the inferencing in the Himalayan club example with different starting points, producing different resolution trees.
- Think of a Prolog implementation of the problem
- Prolog Reference (Prolog by Chockshin & Melish)

Introduction

- PROgramming in LOGic
- Emphasis on what rather than how

Problem in Declarative Form

LogicMachine

Basic Machine

Prolog’s strong and weak points

- Assists thinking in terms of objects and entities
- Not good for number crunching
- Useful applications of Prolog in
- Expert Systems (Knowledge Representation and Inferencing)
- Natural Language Processing
- Relational Databases

A Typical Prolog program

Compute_length ([],0).

Compute_length ([Head|Tail], Length):-

Compute_length (Tail,Tail_length),

Length is Tail_length+1.

High level explanation:

The length of a list is 1 plus the length of the tail of the list, obtained by removing the first element of the list.

This is a declarative description of the computation.

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