TES3111 Oct 2001 Data Structures S-Expression - Symbolic expression. It can be an Atom, a List or a collection of S- Expression enclosed by (…) Atom - String of characters beginning with a letter, digit. Eg: Artificial, intelligence, etc.

TES3111 Oct 2001 Symbols & Lists A symbol is just a string of characters, made up of letters, digits and hyphens. Examples of symbols: jack, jill, what-is-that … etc. Lists consists of zero or more elements enclosed in parenthesis. It looks like this: ‘(element1 element2 …. Elementn)

TES3111 Oct 2001 Symbols and Lists Caution! - Lists must be preceded by a single quote ‘. Otherwise Lisp will evalute the whole expression as a function invocation. Examples of lists: ‘(what day “is today”) ‘(this (is a) nested list) To construct a list, do as use the list function as follows: (list ‘(a b c) (* 2 3 5)) An empty list is represented using () or nil.

TES3111 Oct 2001 Some Lisp Functions (setq x 5) ; assign the value 5 to x (let ((x 100) x) ; temporarily bind x to 100 (* a 9) Note that the semi-colon is use to insert comments. It is not the end of statement indicator as in other languages.

TES3111 Oct 2001 Boolean values True is represented as t and False is represented as nil. Examples of conditionals: (if t “Hello” “Wrong”) (if nil ) (if 1 2 3)

TES3111 Oct 2001 Keywords Keywords are self evaluating symbols. We have already encountered 2 of this kind - t and nil. Any symbols that begins with a colon is a keyword. Examples: :lisp-name-space :what-is-lisp

TES3111 Oct 2001 Numbers Lisp provides 4 types of numbers: –Integers: 12, 9, -12 –Rational numbers: 12/87, 987/10 –Real numbers: 3.142, , e-20 –Complex numbers in the form of #c(r i). Where r is the real part and i is the imaginary part. Examples: #c(1.65, 0.6)

TES3111 Oct 2001 Conversion & Extractions There are functions for converting and extracting components of the 4 kind of numbers. (float 23) ; convert integer to float (truncate ) ; (floor ) (ceiling ) (round )

TES3111 Oct 2001 Arithmetic Please be reminded that Lisp uses the prefix notation for function invocation. x - y - z, in Lisp this is (- x y z) (a * b) + (c*d), in Lisp this is (+ (* a b) (* c d)) (/ 24 7) ( ) Be careful of how you nest the operators.

TES3111 Oct 2001 Conses Try these out: (CAR ‘(a b c)) (CAR (CAR ‘(a b c)) (CDR ‘(a b c)) (CAR (CDR ‘(a b c)) What do these 2 functions do ? A cons is a 2-field record made up of car and cdr. (cons 23 24) (cons (cons 1 2) 3) (car (cons 9 10)) (cdr (cons 10 9))

TES3111 Oct 2001 Using lists as stacks How? By storing a list in a variable and using 2 functions: push and pop. (setq a nil) ; start with an empty stack (push 1 a) (push 2 a) (pop a)