66-2210-01 Programming in Lisp

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66-2210-01 Programming in Lisp

• Introduction to Lisp

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What is Lisp?

• Stands for LISt Processing
• Used for symbol manipulation
• Interactive (interpreted)
• Easy to learn
• Syntax and constructs are extremely simple
• Helps make computers Intelligent

3
Artificial Intelligence

• Sample applications
• Expert Problem Solvers (e.g. Calculus, Geometry,
  etc.)
• Reasoning, Knowledge Representation
• Learning
• Education
• Intelligent support systems
• Natural Language interfaces
• Speech
• Vision

4
Symbolic Expressions

• Data and programs are represented uniformly
• Expression that describes this course
• (course 66221001
• (name (Programming in Lisp))
• (instructor
• (name (Alok Mehta))
• (email (mehtaa_at_cs.rpi.edu)))
• (department (Computer Science)))
• Expression to add 3 2
• ( 3 2) Reverse polish notation!
• Symbolic expressions Atoms and Lists
• Atoms - course, Programming, , 7
• Lists - ( 3 2), (Programming in Lisp)

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Calling Lisp Procedures

• Lisp procedure calls are symbolic expressions
• Represented using lists (like everything else)
• Format of a Lisp procedure call
• (ltprocedure-namegt ltarg1gt ltarg2gt )
• Arithmetic expressions are in Reverse Polish
  Notation
• ( 3 2) Returns 5
• Calls the Lisp procedure with arguments 3
  and 2
• The return value of the expression is 5
• The procedure can take any number of
  arguments
• ( 1 2 3 4) Returns 10

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Overview of Lisp Syntax

• Overview of Lisp Syntax
• ( Left Parenthesis. Begins a list of items.
  Lists may be nested.
• ) Right Parenthesis. Ends a list of items.
• ( ( 3 2) ( 7 8))
• Semicolon. Begins a comment (terminates at
  end of line)
• ( ( 3 2) ( 7 8)) Evaluate ((32)(78))
• " Double Quote. Surrounds character strings.
• "This is a thirty-nine character string."
• Single (Forward) Quote. Dont evaluate next
  expression
• '(Programming in Lisp)
• Examples
• ( 3 2) returns the string "( 3 2) as an
  atom
• ( 3 2) evaluates ( 3 2) and returns 5
• '( 3 2) returns the expression ( 3 2) as a
  list
• Lisp is case-insensitive

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Using Lisp on RCS

• Conventions
• UNIX Prompt
• gt LISP Interpreter prompt
• From a UNIX prompt, start the lisp interpreter
• gcl
• GCL (GNU Common Lisp) Version(2.2) Mon Sep 30
  094544 EDT 1996
• Licensed under GNU Public Library License
• Contains Enhancements by W. Schelter
• gt
• At the Lisp prompt, type your Lisp Expressions
• gt ( ( 3 2) ( 7 8))
• 75
• gt
• Lisp expressions return values
• Return values can be used in other expressions

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Using Lisp on RCS

• Recovering from errors in GCL (q)
• gt ( 4 x)
• Error "x" is not of type NUMBER.
• Fast links are on do (siuse-fast-links nil)
  for debugging
• Error signalled by .
• Broken at . Type H for Help.
• gtgt q
• Executing lisp commands from a file
• gt (load "prog1.lsp")
• Reads and executes the lisp expressions
  contained in prog1.lsp
• Accessing on-line help
• gt (help)
• Exiting from GCL (bye) or CTRL-d
• gt (bye)

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Setf Assigns Variables

• Setf (SET Field) assigns variables (side effect)
• gt (setf a '( 5 3)) Lisps way of saying
  a53
• ( 5 3)
• gt (setf b ( 5 3))
• 8
• Examining variables
• gt a
• ( 5 3)
• gt b
• 8
• Accessing variables
• gt ( 3 b)
• 11
• gt ( 3 'b)
• error
• gt ( 3 a)
• error

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Cons, Remove, First, Rest

• Lists are used to represent knowledge
• gt (setf complang '(C Lisp Java Cobol))
• (C LISP JAVA COBOL)
• Cons (CONStruct) adds an element to a list
• gt (setf complang (cons 'Perl complang))
• (PERL C LISP JAVA COBOL)
• Remove removes an element from a list
• gt (setf complang (remove 'Cobol complang))
• (PERL C LISP JAVA)
• First gets the first element of a list
• gt (first complang)
• PERL
• Rest gets everything except the first element
• gt (rest complang)
• (C LISP JAVA)

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Lists are like boxes NILEmpty
G H
C D
F
A B
E
I
J

• Lists are like boxes they can be nested
• ((( A B ) C D ( E ) ( )) ( F ) G H (((I)(J))))
• NIL is an empty list
• gt (setf messy '(((A B) C D (E) ( )) (F) G H
  (((I)(J)))) )
• (((A B) C D (E) NIL) (F) G H (((I)(J))))
• gt (first messy)
• ((A B) C D (E) NIL)

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First, Rest Revisited

• First returns the first element of a list
• Returns an atom if the first element is an atom
• Returns a list if the first element is a list
• Rest returns all elements of a list except the
  first
• Always returns a list
• Examples
• gt (first '((a) b)) returns (A)
• gt (first '(a b)) returns A
• gt (first '(a)) returns A
• gt (first '( )) returns NIL
• gt (rest '((a) b)) returns (B)
• gt (rest '(a b)) returns (B)
• gt (rest '(a)) returns NIL
• gt (rest '( )) returns NIL

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Getting the second element

• Use combinations of first and rest
• gt (setf abcd '(a b c d))
• (A B C D)
• gt (first (rest abcd))
• B
• gt (first '(rest abcd))
• REST Quote stops expression from being
  evaluated!
• Or, use second
• gt (second abcd)
• B
• third, fourth, , tenth are also defined

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Exercises

• Evaluate
• gt (first '((a b) (c d)))
• gt (first (rest (first '((a b) (c d)))))
• Use First and Rest to get the symbol PEAR
• (apple orange pear grape)
• ((apple orange) (pear grapefruit))
• (apple (orange) ((pear)) (((grapefruit))))
• Other useful exercises
• Text, 2-2, 2-3, 2-4

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Setf Revisited

• Setf
• Format
• (setf ltvar1gt ltvalue1gt ltvar2gt ltvalue2gt )
• Example
• gt (setf x 0 y 0 z 2)
• 2
• Returns
• the value of the last element
• Side effects
• assigns values for symbols (or variables) ltvar1gt,
  ltvar2gt,
• the symbol then becomes an atom that evaluates
  the value assigned to it
• gt x
• 0

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List storage

• Draw List Storage Diagram for
• (setf alist '(A (B (C))))
• Explain semantics of functions
• first, rest, cons, remove
• Draw List Storage diagram for
• ((apple orange) (pear grapefruit))

A
B
C
alist
C
Contents of Address Register (CAR) Old name for
First
B
Contents of Decrement portion of Register (CDR)
Old name for Rest
A
alist
Cons Cell
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Append, List

• Append
• Combines the elements of lists
• gt (append (a b c) (d e f))
• (A B C D E F)
• List
• Creates a new list from its arguments
• gt (list a b (c))
• (A B (C))

A
B
C
D
E
F
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Cons, Setf Push Pop

• Cons has no side effects
• gt (setf complang '(C Lisp Java Cobol))
• (C LISP JAVA COBOL)
• gt (cons Perl complang)
• (PERL C LISP JAVA COBOL)
• gt complang
• (C LISP JAVA COBOL)
• gt (setf complang (cons Perl complang))
• (PERL C LISP JAVA COBOL)
• gt complang
• (PERL C LISP JAVA COBOL)
• Push/Pop - Implement a stack data structure
• Push - shortcut for adding elements permanently
• Pop - shortcut for removing elements permanently
• gt (push complang Fortran)
• (FORTRAN PERL C LISP JAVA COBOL)
• gt (pop complang)
• (PERL C LISP JAVA COBOL

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NthCdr, ButLast, Last

• NthCdr - Generalization of Rest
• Removes the first N elements returns rest of
  list
• gt (setf complang (C Java Lisp Cobol))
• (C LISP JAVA COBOL)
• gt (nthcdr 1 complang) same as (rest complang)
• (LISP JAVA COBOL)
• gt (nthcdr 2 complang) same as (rest (rest
  complang))
• (JAVA COBOL)
• ButLast - Removes the last (n-1) elements
• gt (butlast complang 2)
• (C LISP)
• Last - Returns a list with all but the last
  element
• This function is analogous to first (note
  returns a list though)
• gt (last complang)
• (COBOL)

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Length, Reverse, Assoc

• Length - Returns the number of top-level elements
  of a list
• gt (length (1 2 (3 4) 5)
• 4
• Reverse - Reverses the top level elements of a
  list
• gt (reverse (1 2 (3 4) 5)
• (5 (3 4) 2 1) Note the positions of 3 and 4
• Assoc - Searches sublists for an association
  (alist)
• gt (setf sarah ((height .54) (weight 4.4)))
• ((height .54) (weight 4.4))
• gt (assoc weight sarah)
• (weight 4.4)

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T, NIL, Symbols

• You cant reassign the following symbols
• T True
• NIL Empty List (also means false)
• Symbols can include
• letters, digits, - / _at_ _ lt gt .
• gt (setf mehtaa_at_cs.rpi.edub2-4ac
  funny_variable_name)
• FUNNY_VARIABLE_NAME

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Numbers

• Lisp defines the following types of numbers
• Integers (5, -3)
• fixnum (implementation dependent), bignum
• Ratios (1/3 -- not the same as .333!)
• gt ( 1/3 1/3) returns 2/3
• Floating-Point (3.25)
• short, single, double, long (all are
  implementation dependent)
• Complex
• Format (complex ltreal-partgt ltimaginary-partgt)
• gt (setf i (complex 0 1))
• C(0 1)
• gt ( i i)
• -1

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Misc Math Functions

• ( x1 x2 ) Returns X1 X2
• ( x1 x2 ), (- x1 x2), (/ x1 x2) Computes -, ,
  /
• (float x) converts x to a floating point number
• (round x) rounds a number to the closest whole
  integer
• (max x1 x2 ) Returns the maximum of its
  arguments
• (min x1 x2 ) Returns the minimum of its
  arguments
• (expt x1 x2) Computes first argument (x1) raised
  to the power of the second argument (x2).
• (sqrt x) Computes the square root of x
• (abs x) Computes the absolute value of x

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Review

• Lisp List Processing
• Data and Programs represented using Symbolic
  Expressions
• Atoms, Lists (represented using box analogy or
  cons cells)
• Interpreter functions (load, help, bye)
• Misc. math functions (, -, /, , sqrt, ...)
• Assigning variables (setf)
• List manipulation
• cons, remove, first, rest, append, list
• push, pop
• second, third, , tenth, nthcdr, butlast, last
• length, reverse, assoc
• T, NIL
• Numbers (integers, ratios, floating point,
  complex)