CSCI Programming in Lisp; Instructor: Alok Mehta1 CSCI 2210: Programming in Lisp Introduction to Lisp

CSCI Programming in Lisp; Instructor: Alok Mehta2 CSCI 2210: Programming in Lisp  Instructor  Alok Mehta –Office Hours: After class or by appointment –Home Phone: (518)  Teaching Assistant  Course Web Page   Textbook  Ansi Common Lisp, by Paul Graham; Prentice Hall 1996

CSCI Programming in Lisp; Instructor: Alok Mehta3 Syllabus

CSCI Programming in Lisp; Instructor: Alok Mehta4 Assignments/Grading  Assignments/Grading  HW #1 - Due on January 27 (25%)  HW #2 - Due on February 10 (25%)  HW #3 - Due on March 5 (25%)  Final Exam - March 3 (25%)  Homeworks  Due at 11:59:59 pm on the due date  Total of two grace days are given for late submission  After this, there is a late penalty of 10 points per day

CSCI Programming in Lisp; Instructor: Alok Mehta5 What is Lisp?  Stands for LISt Processing  An old, different programming language  Why study Lisp?  Different, but not outdated  Simple, elegant: syntax and constructs are extremely simple  Encourages "bottom-up" software development  Helps make computers “intelligent”  Easy to write a program that write programs  Programs that can modify themselves based on new knowledge  Artificial Intelligence (AI) uses

CSCI Programming in Lisp; Instructor: Alok Mehta6 Examples of Lisp Applications  Some popular programs written in Lisp –Emacs –Autocad –Interleaf  Other, misc. application areas –Expert Problem Solvers (e.g. Calculus, Geometry, etc.) –Reasoning, Knowledge Representation –Learning –Education –Intelligent support systems –Natural Language interfaces –Speech –Vision

CSCI Programming in Lisp; Instructor: Alok Mehta7 Lisp is Interactive  Lisp is interactive  Toplevel - when lisp is waiting for input  Lisp expressions can be typed into the toplevel  An expression typed into the toplevel is evaluated by the Lisp interpreter > 1 1 >

CSCI Programming in Lisp; Instructor: Alok Mehta8 Atoms and Lists  Atoms  A single element of a particular data type  Examples –1 –3.3 –"This is a String" –ThisIsASymbol  Lists  Lists may contain atoms or other lists  Examples –( ) –(a b (c d) e) –(73 22 b ("A" x) ((((3.14) (4 7) nil (32)))))

CSCI Programming in Lisp; Instructor: Alok Mehta9 Expressions  Expressions  Either atoms or a list  Can represent expressions to be evaluated –Format (operator arg1 arg2 …) –Zero or more arguments –Example > (sqrt 4.0) ; Take the square root of  Can represent data and data structures (course CSCI (name (Programming in Lisp)) (instructor (name (Alok Mehta)) ( (department (Computer Science)))

CSCI Programming in Lisp; Instructor: Alok Mehta10 Expressions are evaluated  We said…  Lisp expressions can be typed into the toplevel  An expression typed into the toplevel is evaluated by the Lisp interpreter  Everything (DATA + PROGRAMS) in Lisp is an expression (i.e. an atom or a list) > 1 1 > (sqrt 4.0) 2.0 > (+ 2 3) ; Note that expressions are in prefix notation! 5

CSCI Programming in Lisp; Instructor: Alok Mehta11 Rules for Evaluation  An expression is evaluated as follows  Each argument are evaluated, from left to right  Call by value occurs for the given operator  Example: > (/ (* 4 6) 3) –/ is the operator; skip this for now –(* 4 6) is the first argument. Evaluate this (note the recursion here) – * is the operator; skip this for now – 4 is the first argument; it evaluates to itself (4) – 6 is the second argument; it evaluates to itself (6) – * is now called, with arguments 4 and 6 passed by value – This evaluates to 24 –We currently have (/ 24 3) –The second argument to / is 3, this evaluates to itself (3) –/ is called with arguments 24 and 3 passed by value –(/ 24 3) is replaced by the value 8, which is returned to the toplevel

CSCI Programming in Lisp; Instructor: Alok Mehta12 More Expression Examples > (/ (* 4 6) 3) 8 > ( ) ; The + operator takes zero or more args 15 > (+) 0 > "Hello" "Hello" > (sqrt (+ 1 3)) 2.0 > (quote (+ 1 3)) ; Exception! Quote = Don't evaluate arg (+ 1 3) > '(+ 1 3) ; shortcut for (quote (+ 1 3)) (+ 1 3)

CSCI Programming in Lisp; Instructor: Alok Mehta13 The Quote Operator  Special Operator  Disobeys the evaluation rule  Does NOT evaluate its argument > (sqrt (+ 1 3)) 2.0 > (quote (+ 1 3)) (+ 1 3) > (quote hello) HELLO > 'hello HELLO > hello Error: Attempt to take the value of the unbound variable `HELLO'. [condition type: UNBOUND-VARIABLE]

CSCI Programming in Lisp; Instructor: Alok Mehta14 Symbols  In the expression below, HELLO is a Symbol > 'hello HELLO  Symbols are used for variables (and other things)  The following expression causes the interpreter to search for a value for the symbol HELLO > hello Error: … [condition type: UNBOUND-VARIABLE]  If the symbol HELLO was bound to something, the value would have been returned > (set (quote hello) 3) ; initialize HELLO to 3 3 > hello 3

CSCI Programming in Lisp; Instructor: Alok Mehta15 Set, Setf Assign Variables  Set and Setf assign variables (side effect) > (set (quote a) '(+ 5 3)) (+ 5 3) > (setf b (+ 5 3)) ; equivalent to (set (quote b) (+ 5 3)) 8  Examining variables > a (+ 5 3) > b 8  Accessing variables > (+ 3 b) 11 > (+ 3 'b) ** error ** > (+ 3 a) ** error **

CSCI Programming in Lisp; Instructor: Alok Mehta16 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 ((3+2)*(7+8)) " Double Quote. Surrounds character strings. "This is a thirty-nine character string." ’ Single (Forward) Quote. Don’t 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

CSCI Programming in Lisp; Instructor: Alok Mehta17 The Story So Far...  Atoms  Lists  Expressions  The Evaluation Rule  Symbols  Set, Setf  Quote (')

CSCI Programming in Lisp; Instructor: Alok Mehta18 How to Run Lisp  Under UNIX (on RCS)  kcl, gcl  Specify in homework which was used  :q to recover from an error  ^D to exit  Under Win '95  Go to  Select Allegro CL Lite for Windows

CSCI Programming in Lisp; Instructor: Alok Mehta19 Using Lisp on RCS  Conventions $UNIX Prompt >LISP Interpreter prompt  From a UNIX prompt, start the lisp interpreter $ gcl GCL (GNU Common Lisp) Version(2.2) Mon Sep 30 09:45:44 EDT 1996 Licensed under GNU Public Library License Contains Enhancements by W. Schelter >  At the Lisp prompt, type your Lisp Expressions > (* (+ 3 2) (+ 7 8)) 75 >  Lisp expressions return values  Return values can be used in other expressions

CSCI Programming in Lisp; Instructor: Alok Mehta20 Using Lisp on RCS  Recovering from errors in GCL (:q) > (+ 4 ’x) Error: "x" is not of type NUMBER. Fast links are on: do (si::use-fast-links nil) for debugging Error signalled by +. Broken at +. Type :H for Help. >> :q  Executing lisp commands from a file > (load "prog1.lsp") ** Reads and executes the lisp expressions contained in “prog1.lsp” **  Accessing on-line help > (help)  Exiting from GCL: “(bye)” or “CTRL-d” > (bye)

CSCI Programming in Lisp; Instructor: Alok Mehta21 How to Write Lisp  Experiment by entering expressions at the toplevel  or, … Write code into a file and save it  Lisp code can be messy  Use an editor with paren matching! –vi: :set sm –emacs: M-x lisp-mode  Load  Reads a file and executes expressions contained in the file, as if you typed them directly at the top level > (load "hello.lsp") t  Load returns the result of the LAST expression evaluated in the lisp file

CSCI Programming in Lisp; Instructor: Alok Mehta22 Constructing New Lists  One way: Use quote operator > (quote (1 2 3)) (1 2 3) > '(1 2 3) (1 2 3)  Another way: Use the list operator > (list 1 2 3) (1 2 3)  When is one better than another? Depends... > '(1 2 (3 (4)) 5) (1 2 (3 (4)) 5) > (list 1 2 (list 3 (list 4)) 5) (1 2 (3 (4)) 5) > '(1 2 (+ 3 4) 5) (1 2 (+ 3 4) 5) > (list 1 2 (+ 3 4) 5) ( )

CSCI Programming in Lisp; Instructor: Alok Mehta23 Cons, Remove, First, Rest  Lists are used to represent knowledge > (setf complang '(C++ Lisp Java Cobol)) (C++ LISP JAVA COBOL)  Cons (CONStruct) adds an element to a list > (setf complang (cons 'Perl complang)) (PERL C++ LISP JAVA COBOL)  Remove removes an element from a list > (setf complang (remove 'Cobol complang)) (PERL C++ LISP JAVA)  First gets the first element of a list > (first complang) PERL  Rest gets everything except the first element > (rest complang) (C++ LISP JAVA)

CSCI Programming in Lisp; Instructor: Alok Mehta24 Lists are like boxes; NIL=Empty G H C D A B F IJ  Lists are like boxes; they can be nested ((( A B ) C D ( E ) ( )) ( F ) G H (((I)(J))))  ‘NIL’ is an empty list > (setf messy '(((A B) C D (E) ( )) (F) G H (((I)(J)))) ) (((A B) C D (E) NIL) (F) G H (((I)(J)))) > (first messy) ((A B) C D (E) NIL) E

CSCI Programming in Lisp; Instructor: Alok Mehta25 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 > (first '((a) b)); returns (A) > (first '(a b)); returns A > (first '(a)) ; returns A > (first '( )); returns NIL > (rest '((a) b)); returns (B) > (rest '(a b)); returns (B) > (rest '(a)); returns NIL > (rest '( )); returns NIL

CSCI Programming in Lisp; Instructor: Alok Mehta26 Getting the second element  Use combinations of first and rest > (setf abcd '(a b c d)) (A B C D) > (first (rest abcd)) B > (first '(rest abcd)) REST; Quote stops expression from being evaluated!  Or, use second > (second abcd) B  third, fourth, …, tenth are also defined

CSCI Programming in Lisp; Instructor: Alok Mehta27 Exercises  Evaluate > (first '((a b) (c d))) > (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))))

CSCI Programming in Lisp; Instructor: Alok Mehta28 Setf Revisited  Setf  Format (setf …)  Example > (setf x 0 y 0 z 2) 2  Returns –the value of the last element  Side effects –assigns values for symbols (or variables),, … –the symbol then becomes an atom that evaluates the value assigned to it > x 0

CSCI Programming in Lisp; Instructor: Alok Mehta29 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 B C A alist Contents of Address Register (CAR) = Old name for “First” Contents of Decrement portion of Register (CDR) = Old name for “Rest” Cons Cell

CSCI Programming in Lisp; Instructor: Alok Mehta30 Append, List  Append  Combines the elements of lists > (append ’(a b c) ’(d e f)) (A B C D E F)  List  Creates a new list from its arguments > (list ’a ’b ’(c)) (A B (C)) ABCDEF

CSCI Programming in Lisp; Instructor: Alok Mehta31 Cons, Setf; Push; Pop  Cons has no side effects > (setf complang '(C++ Lisp Java Cobol)) (C++ LISP JAVA COBOL) > (cons ’Perl complang) (PERL C++ LISP JAVA COBOL) > complang (C++ LISP JAVA COBOL) > (setf complang (cons ’Perl complang)) (PERL C++ LISP JAVA COBOL) > 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 > (push complang ’Fortran) (FORTRAN PERL C++ LISP JAVA COBOL) > (pop complang) (PERL C++ LISP JAVA COBOL)

CSCI Programming in Lisp; Instructor: Alok Mehta32 T, NIL  Lisp uses the symbols T and NIL for boolean values T; True NIL; False (also means Empty List)  These are reserved symbols and can't be re-assigned  Note that NIL is used to mean both an empty list and a false > 'nil NIL > nil NIL The symbol NIL evaluates to itself > '() NIL  Functions that determine truth are called predicates

CSCI Programming in Lisp; Instructor: Alok Mehta33 Predicates: listp  Listp: Is the argument a list? > (listp 'Hello) NIL > (listp '(1 2 3)) T > (listp nil) T NIL is a list with zero elements, but is still considered a list! > (listp '()) T > (listp '(HELLO)) T > (listp (HELLO)) *Error* > (listp '( )) T > (listp ( )) NIL

CSCI Programming in Lisp; Instructor: Alok Mehta34 Predicates: null, not  Null: Is the argument an empty list? > (null nil) T > (null t) NIL > (null 3.4) NIL > (null '(1 2 3)) NIL  Not: Returns the opposite of argument > (not nil) T > (not t) NIL > (not '(1 2 3)) NIL  Not and Null are equivalent

CSCI Programming in Lisp; Instructor: Alok Mehta35 If  If is a macro  Macros do not follow the Lisp evaluation rule  Syntax: (if )  Evaluates the first argument,  If the first argument is true, the second argument is evaluated  Otherwise, if the first argument is NIL, the third argument is evaluated  Example (if (> a b) a b) ; returns maximum of a and b

CSCI Programming in Lisp; Instructor: Alok Mehta36 And/Or  And/Or  Macros (don't follow Lisp evaluation rule)  Lazy evaluation –stop evaluating once you know the answer –may not evaluate all arguments  Return the value of the LAST argument evaluated  And  Returns true if all arguments are true; stops at first false statement  Or  Returns true if any argument is true; stops at first true statement

CSCI Programming in Lisp; Instructor: Alok Mehta37 Procedure Declaration  Simple procedure in C/C++ to square a number double sqr (double x) { return x * x; }  Equivalent function in Lisp (defun sqr (x) (* x x))  Note  C/C++ implementation is strongly typed; Lisp is not  Value returned by procedure is the last expression evaluated (defun sqr (x) (+ 2 3) (* 7 8) (+ x 3) (* x x))

CSCI Programming in Lisp; Instructor: Alok Mehta38 Defun  Defun is a Lisp macro for DEfining FUNctions (defun (...)...)  Side effect –defines a user-defined lisp procedure  Returns the name of the procedure defined  Defun does not evaluate its arguments  Resulting user-defined procedure is used like any other Lisp procedure > (defun sqr (x) (* x x)) SQR > (sqr 5) 25

CSCI Programming in Lisp; Instructor: Alok Mehta39 Output  So far: the toplevel prints return values  But, need a general way to print  E.g. the load procedure only prints the return value of the last expression  Print, Format > (print 3) 3 > (defun verbose-add (a b) (format t "~A plus ~A equals ~A.~%" a b (+ a b)) (+ a b)) > (verbose-add 3 5) 3 plus 5 equal 8 8

CSCI Programming in Lisp; Instructor: Alok Mehta40 Input  Read > (defun askem (prompt) (format t "~A" prompt) (read)) ASKEM > (askem "How old are you? ") How old are you? 3 3

CSCI Programming in Lisp; Instructor: Alok Mehta41 C/C++ Example  C/C++: Convert units of sq. meters to sq. yards #include void main () { const float meters_to_yards = 1.196; float size_in_sqmeters; float size_in_sqyards; cout << "Enter size in square meters: "; cin >> size_in_sqmeters; size_in_sqyards = meters_to_yards * size_in_sqmeters; cout<<"The size in square yards is " << size_in_sqyards << endl; }

CSCI Programming in Lisp; Instructor: Alok Mehta42 Equivalent Lisp Example  Literal Translation > (defun converter () (setf meters_to_yards 1.196) (format t "Enter size in square meters: ") (setf size_in_sqmeters (read)) (setf size_in_sqyards (* meters_to_yards size_in_sqmeters)) (format t "The size in square yards is: ~A~%" size_in_sqyards) ) CONVERTER > (converter) Enter size in square meters: 2.0 The size in square yards is NIL

CSCI Programming in Lisp; Instructor: Alok Mehta43 Better Lisp Function  Using a more lisp-like style of programming > (defun converter (sq_meters) (* sq_meters 1.196)) CONVERTER > (converter 2.0)  Take advantage of toplevel  Users enter expressions; toplevel prints the return value  Functional programming  Avoid variable assignments, side effects  This course covers the Lisp programming language  Programming language constructs, built-in functions  Lisp style, philosophy

CSCI Programming in Lisp; Instructor: Alok Mehta44 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)  Assigning variables (set, setf)  List manipulation –cons, remove, first, rest, append, list –push, pop –second, third, …, tenth  T, NIL, Predicates  If, and, or  Defun