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LISP

What is LISP?

A LISt Processing language

The basic data structure is linked list

A functional programming language Each expression in LISP is a function that returns a value

A good LISP program should avoid too many side effects

An interpretive language

Running LISP programs involves interacting with the LISPinterpreter

LISP programs can also be compiled

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LISP

Why LISP for AI Programming?

LISP supports symbol manipulation better

Symbols are the basic entity of the language

The interpretive nature makes it easy to try anew idea and prototype

Functions are in the form of linked lists

Easier to write learning programs

Historical Reason Most AI programs in the U.S. have been developed in LISP

However, most AI programs in Europe have been developedin PROLOG

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LISP

Functions

Lisp is a functional language

-> sqrt(x)

In lisp this function is:

-> (sqrt x)

Prefix Notation

Interpretive

Primary effect vs. side effects

Example: (print 'hello)

returns

Hello

Hello

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LISP

Read-Eval-Loop

Evaluation process

(+ 3 4)

Parentheses Notification to evaluate

Function name Go get function (in thiscase + is the add function)

space - separatoroperands -- value for function

Parentheses stop evaluation

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LISP

How do I do math in lisp?

Use built in arithmetic functions

Put the arithmetic functions together

Examples:

4 + 5

4 + 5 + 6

(4 + 5 + 5) (2 + 2)

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LISP

Arithmetic Functions

(+ numbers)

(- numbers)

(* numbers)(/ numbers)

(1+ number)

(1- number)(abs number)

(acos number)

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LISP

(atan )

(cos ..

(exp number

(expt n1 n2)

(gcd numbers)

(log

(max(min

(random number) ---between 0 and number specified

Etc.

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LISP

Back to the Examples:

Examples:

4 + 5 -> (+ 4 5)

4 + 5 + 6 -> (+ 4 5 6)

(4 + 5 + 5) (2 + 2) (* (+ 4 5 5) (+ 2 2))

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LISP

Exercises

Evaluate the following:

(+ 3 2)

(+ 4 (/ 4 2 ) (* 2 4))

Create functions for:

(3 + 4 + 5 + 6)(- 2 4)

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LISP

How do I process words,characters, symbols?

Examples of problems:

Is the word kathy in the list that the user

entered?How many words are there in the sentence?

Answer: I use built-in lisp functions to put liststogether and take them apart.

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LISP

Atoms and Lists

Atoms

Numbers, symbolic atom (anything not a number usually a listof characters

ListsAnything with parentheses around it.

()

(a)

(this is one too)

(a list of (two) lists)

(a (very (very (very (inscrutable) list)))

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LISP

The Building Block of Lists:

The CONS Cell A CONS cell contains two pointer: CAR and CDR.

The CAR (also called first) pointer usually points to avalue (e.g., a symbol, or a number).

The CDR (also called rest) pointer usually points to thenext CONS cell in a linked list.

CAR CAR

CDRCDR

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LISP

Basics of Lists

A linked list always terminates by having theCDR of the last CONS cell point to a specialsymbol: NIL.

Example: ( John loves Mary )

John loves Mary

NIL

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LISP

A Special Symbol: NIL

NIL represents an empty list.

NIL is a terminator of a list.

A list is usually built by inserting its elementsinto NIL in the reverse order .

NIL can also represent false''.

The special symbol representing ``true'' is T.

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LISP

Creating a CONS cell

The function CONS returns a newly createdCONS cell.

The function takes two arguments.

The CAR pt of the new cell points to the first argument.

The CDR pt of the new cell points to the second argument.

Example: (CONS Mary NIL) returns

Mary

NIL> ( MARY )

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LISP

Using CONS to insert an itemto the front of a list

(CONS )

Example: (CONS 'John (CONS 'loves (CONS'Mary NIL))) returns

* Third CONS call returns ( JOHN LOVES MARY )

* Second CONS call

returns ( LOVES MARY )

* First CONS call

returns (MARY )

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LISP

Nested Sublists

A sublist is a list pointed by a CAR pointer ofa CONS cell.

Example: ( John loves ( play tennis ) )

John loves

NIL

NIL

play tennis

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LISP

Using CONS to Insert Sublists

When CONS inserts a list into another list.The former becomes a sublist of the latter.

Example: (CONS '(Amazon BN) '(sellsbooks on Internet)) returns

->( ( AMAZON BN ) SELLS BOOKS ONINTERNET )

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LISP

Taking lists apart

(CAR ) returns the first element of thelist.

(CDR ) returns the remaining list (i.e.,

everything except the first element).

Example: (CAR '(John loves Michelle) ) returns

> JOHN

Example: (CDR '(John loves Michelle) )

returns> (LOVES MICHELLE)

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LISP

Quote

Quote symbol is a short hand for a functionQUOTE.

(QUOTE ) QUOTE is a special function that prevents

LISP from evaluating its argument.

QUOTE returns the argument literately.

Example: (quote (dummy-fn 2))

==> (DUMMY-FN 2)

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LISP

Other List Creating Functions

(LIST ... ) returns alist whose first element is , secondelement is , ...., Nth element is.

Example: (LIST 'John 'loves Michelle) returns(JOHN LOVES MICHELLE)

Example:(LIST 'John 'loves (LIST AI research)) returns(JOHN LOVES (AI RESEARCH))

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LISP

Concatenating Lists

(APPEND ) returns a list that isthe result of concatenating and.

Example: (APPEND '(Orange Apple) '(Grape))returns(ORANGE APPLE GRAPE)

APPEND can also be used to concatenate

more than two lists.

Example: (APPEND '(play) '(tennis)'(football baseball) ) ) ) returns(PLAY TENNIS FOOTBALL BASEBALL)

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LISP

Evaluation

LISP executes/interprets an expressionthrough an evaluation procedure.

Example:

(+ 3 5) evaluates to ==> 8

(CONS A NIL) evaluates to ==> (A)

3 evaluates to ==> 3A evaluates to ==> A

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LISP

Basic Evaluation Rules

A number evaluates to itself

A symbol evaluates to its value.

SETQ assigns a value to a symbol

A list is evaluated by

treating the first element as a function

evaluating each arguments of the function in a left-to-right

order

An expression preceded by a quote symbol evaluates to the expression itself.

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LISP

Assignment and Binding

A symbol (or variable) can be assigned avalue (called its binding) using SETQ.

(SETQ )

Example: (SETQ A (A B C)) ==> (A B C)

A evaluates to ==> (A B C)

Evaluating a symbol that does not have a

value assigned (i.e., no binding) causes errorEx: B evaluates to ==> Error: Unbound

Variable

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LISP

All other functions do NOTchange the bindings

In particular, CAR and CDR are non-destructive.

> (setq my-friends (Superman Batman Robin) )

(Superman Batman Robin)

> (car (cdr my-friends))

Batman> my-friends


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LISP

All other functions do NOTchange the bindings (cont.)

CONS does not change bindings either.

> (CONS J-Bond my-friends)

(J-Bond Superman Batman Robin)

> my-friends


> (setq my-friends (CONS J-Bond friends) )


> my-friends


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LISP

Exercises

Which of the following are atoms, which lists, which bothand which neither?

1. nil3. (expt 10 3)4. (a b)5. 646. t7. (tom jerry donald micky)

8. (+ 3 5 6)

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LISP

What is returned by each of the followingexpressions (assume they are evaluated inthe given order)?

a. (setq trek '(picard riker laforge worf))

b. (cons 'data trek)

c. (append data trek)

d. trek

d. (setq trek (cons 'data trek))

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LISP

Defining My-Own Functions

A function is defined using DEFUN

(DEFUN ( ...) ... )

All arguments are passed by value.

The body of the function may contain anynumber of expressions (i.e., function calls).

The function returns the value returned by thelast expression.

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LISP

Defining A Function

(defun square (x)

(times x x) )

(defun add-friend (new-friend friends)

(cons new-friend friends) )

After defining it, I call the function!!!!

-> (square 5)

25

-> (add-friend B-Gates my-friends)

(B-Gates J-Bond Superman Batman Robin)

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LISP

Changing a symbols binding does notchange its function def.

>(setq cons list)

LIST

>(setq b (cons cons nil))

(LIST)

> cons

LIST

> (cons cons b)

(CONS LIST)

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LISP

Predicates Checking to see ifsomething is true.

Functions that return ``true (i.e., T) or``false (i.e., NIL).

type-testing predicates

(NULL ) returns T if is NIL(empty list), otherwise NIL.

(LISTP ) returns T if is a list,otherwise NIL.

(ATOM ) returns T if is an atom(i.e., a symbol, a number, or NIL).

(NUMBERP ) returns T if is anumber

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LISP

Equality-testing Predicates

(EQ ) returns T if twoarguments are two identical atoms.

(EQUAL ) returns T if two

arguments are identical atoms or identicallists.

> (EQ (Banana Apple) (Banana Apple))

NIL> (EQUAL (Banana Apple) (Banana Apple))

T

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LISP

Other Predicates

(MEMBER ) returns the elements if isan atom that is a top-level element of.

> (setq my-friends (MacGyver (Batman Robin)) )

> (member MacGyver my-friends)

macgyver

> (member Batman my-friends)

NIL

> (member (Batman Robin) my-friends)

NIL

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LISP

Conditional Expression

COND is an N-branch conditional expression

(COND ( ... )

( ... )

...

( ... ) )

Each test is evaluated sequentially until a testreturns true.

Expressions following that test will beexecuted.

COND returns the value returned by the lastexpression associated with the test.

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LISP

Example of COND

(defun select-character (enemy)

(cond ( (eq enemy Penguin)

Batman)( (eq enemy Catwoman)

J-Bond )

( (eq enemy Black-Knight)

(White-Knight King-Arthur) ))

)

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LISP

Terminates a COND with a Tcondition

(defun select-character (enemy)(cond ( (eq enemy Penguin)

Batman)( (eq enemy Catwoman)

J-Bond )( (eq enemy Black-Knight)

(White-Knight King-Arthur ) )( T ; for all other enemies

SuperMan) ; ask Superman for help)

)

LISP

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Building More ComplexConditions

Simple tests can be combined into a complex oneusing AND, OR, NOT.

(AND ... ) evaluates tests in a left-to-right order. It returns NIL when it encounters the

first test that evaluates to NIL (remaining tests are notevaluated). If all the tests return non-NIL, AND returnsthe value of the last test (i.e., testn).

(defun safe-caar ( L )

(and (listp L)

(listp (car L))(car (car L) )

) )

LISP

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More Examples of AND

> (safe-caar A)

NIL

> (safe-caar ( ( A ) ) )

A

> (caar A)

Error!

LISP

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OR and NOT

(OR ... ) evaluates testsin a left-to-right order. It returns T when itencounters the first test that evaluates to T

(remaining tests are not evaluated). If all thetests return NIL, OR returns NIL.

(NOT ) returns T if is NIL, returnsNIL if is non-NIL.

> (NOT (NULL (A B C) ) )

T

LISP

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Using AND, OR in COND

(defun Evaluate-feeling ( sentence )

(cond ( (OR (member hate sentence)

(member dislike sentence))

hatred)( (AND (member I sentence)

(member feel sentence) )

self-centered )

( T

happy)

) ; end of cond

) ; end of defun

LISP

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problems

Rewrite the following in lisp

23 * 2

45/9 3

50 1020

1.0/2.0

5 * [20-12/2]

LISP

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(setq law1 ((a robot) may not (injure a (human) being)(((or))) (through inaction) allow a (human being) (((to)come) to) harm))

Give me the series of cars and cdrs to retrieve the

following

(a robot) may

Not (injure a (human) being)

(human) (((or)))Or allow

Harm nil

LISP

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Write a function that is invoked as:

(asc-sqr )

returns t if a square = b and b square = c

Write a function to determine a students lettergrade given a number

(grade )

returns A= 100-90; B= 89-80;etc.

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