TES3111 October 2001

Artificial Intelligence

LISPLISP

TES3111 October 2001

Lisp resources

• We will use an implementation of LISP called Allegro Common LISP.

• Different LISP interpreters include - Eg interpreter: Allegro LISP, Harlequin LISP, Corman Lisp.

Text books:ANSI Common LISP, P.Graham, Prentice Hall, 1995

(recommended)Common LISP:the language, G.L. Steele, Digital Press,

1990 (2nd Edition)

TES3111 October 2001

Lisp resources-cont

Common LISP: A gentle introduction to Symbolic Computing, David Touretzky, Addison Wesley, 1990.

Paradigms of Artificial Intelligence Programming: Case Studies in Common LISP, Peter Norvig, Academic Press/Morgan Kaufmann, 1992.

TES3111 October 2001

Useful websites for LISP

• Allegro Lisp download – http://www.franz.com/downloads/

• Association of LISP users– http://www.alu.org

• Common LISP Open Code Collection– http://clocc.sourceforge.net

• AI special interest group of the ACM -– http://www.sigart.acm.org

TES3111 October 2001

1. Lisp is interactive

• There is an interpreter that evaluates inputs. An input is processes in 3 steps:1. Reads input and construct expression from the input.

2. Evaluates the expression for meaning.

3. Prints the results of the evaluation, including signaling of erros if necessary.

• These 3 steps can be customized by the programmer.

TES3111 October 2001

2. Lisp is a dynamic language

• Programs are developed incrementally, by making small changes to the source code.

• Interpreter evaluates the changed definitions and then immediately run the results.

• New definitions and data structures can be added at any time.

• This features are ideal for prototyping.

TES3111 October 2001

3. Lisp has symbols

• Symbols are the basic type of data in use.

• Symbols are used to build bigger, more complex expressions.

• Example of symbols: – HELLO

– 23-worldofsports

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4. LISP has lists

• Lists are delimited using parenthesis (…). Anything can be placed in a list, including other lists (nested lists). For example:(1 orange 2 3)

(once (upon a) time)

• Empty list is represented as ()• Caution: elements within a list are separated with

a white space, and NOT a comma ,

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5. Lisp classifies data

• It does not classify variables. • A variable is just a symbol. It can hold any type of

value. • A variable do not have to be declared before it is

used. • Lisp defines different types of data (rather than

defining different types of variables)

TES3111 October 2001

5. Lisp classifies data - contd

Lisp Expression

number symbol sequence

integer

float

ratio

keyword

list vector

string

TES3111 October 2001

5. Lisp classifies data - contd

• Integer - a counting number like 1, 2 ,3 …100, -23

• float - real number. Example 1.59, -100.3

• ratio - a fraction, example 99/23, 4/5

• symbol - a sequence of alphanumeric characters, eg: BOO, ID4…

• keyword - a symbol prefixed with a colon. Eg- :BOO, :ID4

• list - a collection of zero or m ore expressions inside (..)

• vector - 1 dimensional collection of expressions in sequential memory

• string - a vector of 0 or more characters inside “ ”

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6. Lisp uses prefix notation

• Operators appear in front of their operands.

• The infix (10 + 3) is written as (+ 10 3)

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7. Lisp is functional

• Lisp functions take data, operates on it, and return the results. The returned results are called function values.

• Functions can return any number of values.

• To call a function, place it as the first element of an input list, followed by its operands. Example:(+ 100 99 88)

(setf x (+ 2 3))

• All operations are done through functions

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8. Programs and data

• Lisp makes no distinction between programs and data.

• A program can be treated as a set of instruction or as a list of symbols.

• This makes it possible to write programs that generate another program, or programs that analyze other programs.

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9. Lisp evaluation is easy to understand

• Rule 1 : If an expression is a constant, the interpreter will return the value of

the constant. No more rules apply.

Examples: ‘socrates, 4.5

• Rule 2:If Rule 1 is not applicable, and the expression is a symbol, then Lisp

treats the symbol as a variable and no more rules are considered. If the variable has a value, Lisp will return that value. Otherwise it will report an error.

TES3111 October 2001

9. Lisp evaluation is easy to understand

• Rule 3: If Rule 1 and Rule 2 do not apply and the expression is a LIST, then

Lisp treats it as a function call and no more rules are considered.

– You should at this point remember that the first element in the list is assumed to be a defined a function. The remaining elements are data. Each expression in the data is evaluated left to right.

• Rule 4:If Rules 1 to 3 do not apply, then there is an error!

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10. Lisp is easy to learn???

• How to study and program in Lisp?– Read the first few chapters of a good introductory Lisp

book.

– Read up the definition of each Lisp function that you encounter.

– Start developing a good programming style from the very start!

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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, 31416..etc.

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Data Structures

List - a collection of S-Expression enclosed by ( …. ). Eg:(One of these days)

(one ((two) three) four)

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Basic Operations

A LISP program contains functions applied to its arguments.

Functions return LISP objects. 2 types of functions - PREDICATE and

COMMANDS

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Predicate

A function that tests for some condition involving its arguments and returns– Nil if the condition is FALSE (Nil stands for FALSE)

– True for any other case. In LISP TRUE is represented using a special LISP variable T.

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Command

• A command performs operations on its arguments and returns an S-expression.

• Format of a command is:

(<Command> Arg1 Arg2 … Argn)

Note: All commands and predicates must be enclosed in (…). To differentiate from lists… a list is preceded with a single quote ‘

Eg : (CAR a b c) versus ‘(CAR a b c)

TES3111 October 2001

Function - CAR

CAR - returns the first element of a list. Examples:

• (CAR ‘(a b c d e))• (CAR ‘((an orange) a day))• (CAR '(1 (2 3 (4 5)) 6))• (CAR ‘())

Note that in the last example, the argument to CAR is a null-list.