CS 380 ARTIFICIAL INTELLIGENCE INTRODUCTION TO LISP · LISP introduction Functional language: • Avoids changing state: i.e., variables have one value, and one value only (you don’t

CS 380ARTIFICIAL INTELLIGENCE

INTRODUCTION TO LISPSantiagoOntañó[email protected]

LISPintroduction

LISPintroduction

LISP1958

LISPintroduction

SHRDLU:https://www.youtube.com/watch?v=bo4RvYJYOzI

(ReadingaboutSHRDLUasakidmademewanttostudyAI!)

LISPintroduction

Functionallanguage:• Avoidschangingstate:i.e.,variableshaveonevalue,

andonevalueonly(youdon’tchangevalueofvariablesonceyouassignthem).• Inreality,youcan(Lispisnotpurelyfunctional).But

thisshouldbeavoided.• ExpressionsvsStatements:infunctionalprogramming,

thereturnvalueofafunctionshoulddependONLYonthevalueofitsparameters.Unlikeinimperativelanguages,wheretherecanbeglobalstate.

• RecursionvsIteration:loopingisimplementedusingrecursion,insteadofiteration.

LISPintroduction

Inventedin1958byJohnMcCarthy(sameguywhocoinedtheterm“ArtificialIntelligence”)

Whatdoes“LISP”standfor??LISt Processing

Simplerepresentationandmanipulationoflists‘(A E I O U)

LISPintroduction

Whatdoes“LISP”standfor??

LISt Processing

Simplerepresentationandmanipulationoflists(let ((vowels ‘(A E I O U))))

A E I O U

NIL

LISPontux.cs.drexel.edu

MIT/GNUSchemeisinstalledontux.cs.drexel.edumit-scheme

Toquit(quit) or (exit)

Youcanloadfilesfromthecurrentdirectory(load “filename”)

CommonLispisinstalledontux.cs.drexel.educlisp

Toquit(quit) or (exit)

Youcanloadfilesfromthecurrentdirectory(load “filename”)

LISPintroduction

peanut

NIL

Primitiveelements:atoms andlists• atoms:peanutbutterjelly“astring”58• lists:(atomsand/orlists)

(peanut butter jelly)

(peanut butter (marshmallow jelly))

NILbutter

marshmallow jelly

Math

AllfunctioncallsinLISPfollowthepattern(function_name arg1 arg2 arg3 ...)

> (+ 2 3)5> (- 5 7)-2> (* 6 7)42

Imperativelanguage:f(x)LISP:(fx)

meansapplyfunctionftoargumentx

Lists

Let’ssaywewanttocreateastaticlistwithnumbers1to5:> (print (1 2 3 4 5))???

Inalist,firstelementisexpectedtobeafunctionwhichusesremainingelementsasarguments.

Ifthelistis“data”insteadofa“functioncall”suppressevaluationwith‘(quote)

Lists

Let’ssaywewanttocreateastaticlistwithnumbers1to5:> (print (1 2 3 4 5))Errorsince1isnotafunction!!!

> (print ‘(1 2 3 4 5))

Inalist,firstelementisexpectedtobeafunctionwhichusesremainingelementsasarguments.

Ifthelistis“data”insteadofa“functioncall”suppressevaluationwith‘(quote)

Lists

Inalist,firstelementisexpectedtobeafunctionwhichusesremainingelementsasarguments.

Ifthelistis“data”insteadofa“functioncall”suppressevaluationwith‘(quote)

Anotherwayisusingthefunction“list”,whichcreatesalistwithallofitsremainingarguments,e.g.: (list 1 2 3)

Lists

Inalist,firstelementisexpectedtobeafunctionwhichusesremainingelementsasarguments.

Ifthelistis“data”insteadofa“functioncall”suppressevaluationwith‘(quote)

Anotherwayisusingthefunction“list”,whichcreatesalistwithallofitsremainingarguments,e.g.: (list 1 2 3)

So,whatifwewanttocreatealistwith3numberslikethis?:(print ‘(1 2 (+ 10 20)))

Lists

Inalist,firstelementisexpectedtobeafunctionwhichusesremainingelementsasarguments.

Ifthelistis“data”insteadofa“functioncall”suppressevaluationwith‘(quote)

Anotherwayisusingthefunction“list”,whichcreatesalistwithallofitsremainingarguments,e.g.: (list 1 2 3)

So,whatifwewanttocreatealistwith3numberslikethis?:(print ‘(1 2 (+ 10 20)))

Doesn’tworkasexpectedsincethequotepreventsevaluation!(print (list (1 2 (+ 10 20))))

Lists(extractinformation)

Getthefirstelement (head)(car '(5 6 7 8))

Gettherestofthe list(tail)(cdr '(5 6 7 8))

A E I O U

NILvowels:

> (car vowels)A> (cdr vowels)(E I O U)

> (car (cdr vowels))E

> (cadr vowels)E

Lists(extractinformation)

Getthefirstelement (head)(car '(5 6 7 8))

Gettherestofthe list(tail)(cdr '(5 6 7 8))

A E I O U

NILvowels:

> (car vowels)A> (cdr vowels)(E I O U)

> (car (cdr vowels))E

> (cadr vowels)E

“car” and“cdr” arefor“hardcore”lisppurists.Ihavealwaysused“first”and“rest”,whichare

moreintuitivetome.

Lists(concatenation)CONS

(let* ((L '(peanut butter jelly))(J (cons ‘apple L)))

(print J))

GivenalistLandanitemx(eitheranatomoralist)(consxL)returnsanewlistwithxasfirstelementandLasrest(Lisunaffected)

> (apple peanut butter jelly)

peanut

NIL

butter jelly

L

apple

J Itdoesnot createanewcopyofalistargument

Code=Data

Lispdoesnotdistinguish“code”from“data”.Forexample:

(let ((a ‘(+ 3 4))(print a) ;usingthecontentofaasdata(eval a) ;usingthecontentofaascode

)

Youcaneasilywritecodethat:• writes/modifiesitself• passfunctions(orevenpartiallydefinedfunctions)as

parameters• etc.

Aboutequality

eq:comparestwoatoms,orwhethertwopointerspointtothesamelocation.equal:compareswhethertwostructureshaveidenticalformandvalues.

(setf x '(a (b c) 1 2 3)) (setf y (car (cdr x))) (setf z (cdr x)) (setf w (car z)) (eq y w) ‘returns T (eq y ‘(b c)) ‘returns NIL (equal y ‘(b c)) ‘returns T

Iamusing“setf”(assignment) inthisslide, forsimplicity, inrealLispprogram,youshould

minimizetheuseof“setf”,anduse“let” instead.

Aboutequality(setf x '(a (b c) 1 2 3)) (setf y (car (cdr x))) (setf z (cdr x)) (setf w (car z)) (eq y w) ‘returns T (eq y ‘(b c)) ‘returns NIL (equal y ‘(b c)) ‘returns T

NIL

NIL

a

b c

1 2 3

x

z

yw

NIL

b c

Additionalfunctions

Functiondoesnotneedtobegivenaname.( (lambda (x) (+ x x)) 7)

Definelocalvariablesforuseinanexpression(let ((a 4)

(b 7)) (+ a b))

Applyafunctiontoeveryelement inalist.(defun square(x)(*xx))=>SQUARE(mapcar 'square'(12345))=>(1491625)

Conditionalexpressions

Theifstatementtakesthree inputs. Ifthefirstargumentistrueisreturnsthesecondotherwise isreturnsthethird.

(if (< 1 3) 4 5)

Thecond extendstheifstatementtoanunlimitednumberofcases.

(cond((< 4 5) 5)((> 4 7) 10)(T 11)

)

CASE(conditional)

CASEislikeC’s“switch”Thevij args aren’tevaluatedotherwise isoptionalandislikeC’sdefault

(case x ((v11 v12 . . .) expr11 expr12 ...)((v21 v22 . . .) expr21 expr22 ...)...(otherwise expr1 expr2 ...))

(setf day4)(caseday

(1(formatt"~%Monday"))(2(formatt"~%Tuesday"))

…(7(formatt"~%Sunday")))

DefinefunctionsThedefun commandcanbeusedfordefiningfunctions.

(defun name (arg list)expression)

Noloops(nottrue).Everything asrecursivefunction.Example:.

(defun fib (n)(if (< n 3)

1(+ (fib (- n 1))

(fib (- n 2)))))

Onceafunctionisdefinedwecanapplyittoarguments.(fIB 6) <- not case sensitive!!!

Recursionreview

DefineLASTfunction

(cond(<test> <result>)(<test> <result>)...(T <result>))

(defun <name> (<arg list>)<expression>)

(NULL x) True iff x is NIL

Recursionreview

Define(LASTL)function

(defun LAST (L)(cond ((NULL L) NIL)

((NULL (cdr L)) L)(T (LAST (cdr L)))))

Define(MEMBERXL)function(useEQUAL)

Recursionreview

Define(LASTL)function

(defun LAST (L)(cond ((NULL L) NIL)

((NULL (cdr L)) L)(T (LAST (cdr L)))))

Define(MEMBERXL)function(useEQUAL)(member2'(123))=>(23)

(defun MEMBER (X L)(cond ((NULL L) NIL)

((EQUAL X (car L)) L)(T (MEMBER X (cdr L)))))

Numericfunctions

+,*,/ plus,times,divide (/ (* 2 3 4) (+ 3 1)) ⇒ 6

- minus (- (- 3) 2) ⇒ –5

sqrt squareroot (sqrt 9) ⇒ 3

exp,expt exponentiation (exp 2) ⇒ e2(expt 3 4) ⇒ 81

log logarithm (log x) ⇒ ln x(log x b) ⇒ logb x

min,max minimum,maximum (min -1 2 -3 4 -5 6) ⇒ –5

abs,round absoluteval,round (abs (round -2.4)) ⇒ 2

truncate integerpart (truncate 3.2) ⇒ 3

mod remainder (mod 5.6 5) ⇒ 0.6

sin,cos,tan trigfuncs (radians) (sin (/ pi 2) ⇒ 1.0

Listfunctions

nth nthelement,nstartsat0

(nth 2 '(a b c d)) ⇒ c

length #ofelements (length '((a b) c (d e))) ⇒ 3

cons concatenation (cons 'a '(b c d)) ⇒ (a b c d)(cons '(a b) 'c) ⇒ ((a b) . c)

list makealist (list (+ 2 3) '(b c) 'd 'e)⇒ (5 (b c) d e)

append appendlists (append '(a) '(b c) '(d))⇒ (a b c d)

(append '(a) '(b c) 'd)⇒ (a b c . d)

reverse reversealist (reverse '((a b) c d)) ⇒ (d c (a b))

Predicates

numberp,integerp,stringp,characterp,evenp,oddp

Testwhetherarg isanumber,integer,string,character,etc.

(numberp 5.78) ⇒ T(integerp 5.78) ⇒ NIL(characterp #\a) ⇒ T

listp,atom,null,consp

Test whetherarg isalist,atomempy/noempty list

(listp nil) ⇒ T(consp nil) ⇒ NIL

<,<=,=,>=,> numericcomparisons arg mustbe anumberstring<,string<=, ... stringcomparisons args must bestring or chareq,equal equalitytests (setf x '(a))

(eq x x) ⇒ T(eq x '(a)) ⇒ NIL(equal x '(a)) ⇒ T

and,or,not logicalpredicates;notandnullareidentical

(not (evenp 8)) ⇒ NIL(and 3 'foo T) ⇒ T

Sequentialexecution(progn e1 e2 ...en)evaluatese1,e2,...,en,andreturnsthevalueofen(prog1e1 e2 ...en)evaluatese1,e2,...,en,andreturnsthevalueofe1

let*assignsinitialvaluessequentially(let* ((x1 v1) (x2 v2) (x3 v3))

e1 e2 ... en)

(let ((x1 v1))(let ((x2 v2))

(let ((x3 v3))e1 e2 ... en)))

Formattedoutput(format <destination> <control-string> <args>) islikeprintf inC

> (setf x "foo")> (format t "~a is ~a~%" 'x x)

X is fooNIL

destinationiswheretosendtheoutputt⇒ sendtostandardoutput,thenreturnNIL

control-stringislikeaprintf controlstringinC~%isanewlinelike\ninC~amatchesanyLispexpression

Loops(dotimes (i num [value]) expressions)executesexpressionswithi =0,...,num −1,thenreturnsvalueorNIL(dolist (x list [value]) expressions)executesexpressionswithx=eachelementoflist,thenreturnsvalueorNIL(return value) returnsvaluefromthemiddleofaloop

(setq result nil)(dotimes (foo 5 (reverse result))

(push foo result)) ⇒ (0 1 2 3 4)

(setq result nil)(dolist (foo '(a 1 b 2 "stop here" 3 z 33))

(if (stringp foo) (return result))(push foo result)) ⇒ (2 B 1 A)

Loops(do ((i1 start1 incr1) . . . (in startn incrn))(termination-test [expressions to evaluate at termination])expression1. . .expressionn)Somewhat likeC’s“for”,buttheiterationvariablesarelocal,andaresetsimultaneously.Tosetthemsequentially, replacedowithdo*Unfortunately, thesyntaxisabitpainful

(setf c 0)(do ((a 1 (+ a 1)) ; a = 1, 2, 3, ...

(b '(1 10 3 2) (cdr b))) ; take successive cdrs((null b) c) ; if b is empty, return c

(setf c (+ c (expt (car b) a)))) ; add x^a to c

⇒ compute 11 + 102 + 33 + 24 = 144

Loops(loop [loop clauses])iterationmacrowithahugenumberofoptions

Becareful!complexcasescanbehardtounderstand(seeANSICommonLisp,pp.239-244)

Butsimplecasesareeasiertounderstandthandois:

(setf c 0)(do ((a 1 (+ a 1))

(b '(1 10 3 2) (cdr b))) ((null b) c)

(setf c (+ c (expt (car b) a))))

⇒ compute 11 + 102 + 33 + 24 = 144

(loop for a from 1 by 1for b in '(1 10 3 2)sum (expt b a))

Lisp• Functionalprogramminglanguagemixedwithlotsofimperative

functionality.

• Data=code

• Veryconvenienttomanipulatesymbols,lists,etc.

• Syntaxwasdefinedinthe50s,wherecomputingpowerwaslimited.So,itis“computerfriendly”,butnotvery“humanfriendly”unlesswepaycloseattentiontoourprogrammingstyle.• Lispletsuswriteverynastylookingprogramsifnotbeing

careful!!!

• UsedinmostearlyAIsoftware