Page 72 - DCAP310_INTRODUCTION_TO_ARTIFICIAL_INTELLIGENCE_AND_EXPERT_SYSTEMS

Introduction to Artificial Intelligence & Expert Systems

Notes The second argument, (1st-number 2nd-number), is a list of what are called the parameters.
These are symbols representing the numbers we are going to refer to later in the procedure.
The rest of the definition, (/(+ 1st-number 2nd-number) 2), is called the body of the procedure.
It tells LISP how to calculate the value of the procedure, in this case sum the numbers and divide
by 2.

The words you use for the parameters are arbitrary, as long as they match the words you use in
the procedure definition. So you could equally well have written the average procedure as
follows:
(defun average (a b)
(/(+ a b) 2))
To define it we can type the definition at the LISP prompt:
CL-USER > (defun average (1st-number 2nd-number)
(/(+ 1st-number 2nd-number) 2))
AVERAGE
Alternatively you could type the definition into the LISP Editor and select Compile Buffer to
evaluate it.
We can try it out with:
CL-USER 13 > (average 7 9)
8
or:
CL-USER 14 > (average (+ 2 3) (+ 4 5))
7
Remember that the arguments are evaluated before they are given to the procedure.

Procedures Without Parameters

A procedure doesn’t have to have any parameters. Here’s a procedure dice that returns a random
dice throw from 1 to 6:
(defun dice ()
(+ 1 (random 6)))
To call the procedure you simply write:
CL-USER 10 > (dice)
5

Example 1:
Objective: Write a program showing use of LISP arithmetic functions.
Solution:

Function Call Value Returned
(+ 3 5 8 4) 20
(- 10 12) -2

(* 2 3 4) 24
(/25 2) 12.5

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