comp 205 – week 11 dr. chunbo chu. intro lisp stands for “list process” invented by john...

COMP 205 Week 11 Dr. Chunbo Chu

Intro Lisp stands for LISt Process Invented by John McCarthy (1958) Simple data structure (atoms and lists) Heavy use of recursion Interpretive language Functional language Variations Frantz Lisp (80s) Common Lisp (de facto industrial standard)

Why Lisp? Because its the most widely used AI programming language Because AI researchers and theoreticians like using it Because its good for writing production software (Graham article) Because its got lots of features other languages dont Because you can write new programs and extend old programs really, really quickly in Lisp

The IDE Install Lisp in a Box loop read in an expression from the console; evaluate the expression; print the result of evaluation to the console; end loop. REPL

Atoms: numbers: (real 1.0, integer 1) symbols: a consecutive sequence of characters (no space) e.g., a, x, price-of-beef. two special symbols: T and NIL for logical true and false. strings: a sequence of characters bounded by double quotes e.g., "this is red". (Note: LISP is case insensitive) Lists: a list of atoms and/or lists, bounded by "(" and "), e.g., (a b c), (a (b c)) top elements of a list example: top elements of list (a b c) are a, b, and c top elements of list (a (b c)) are a and (b c) nil: empty list, same as (). Valid objects (S-expressions)

2. Function calls also a list use prefix notation: (function-name arg1... argn) returns function value for the given list of arguments functions are either provided by Lisp function library or defined by the user. Examples: >(+ 1 3 5) 9 >(/ 3 5) 3/5 >(/ 3.0 5) 0.59999999999999998 >(sqrt 4) 2

Sqrt + *

First Lisp program ( defun hello () (format t Hello World!)) Use Emacs to create a new file (C-x f)containing the above code. Save the file (C-x C-s) under name hello.lisp Switch back the SLIME buffer and load the program by: (load hello.lisp) Run your program: (hello)

exit quote = `

Atoms numeric fractions floating point literal atoms Boolean values other symbols strings

Lists NIL = ()

Function calls evaluation of functions

setf setf more general than setq binding

3. Evaluation of S-expression 1) Evaluate an atom. numerical and string atoms evaluate to themselves; symbols evaluate to their values if they are assigned values, return Error, otherwise; the values of T and NIL are themselves. 2) Evaluate a list - evaluate every top element of the list as follows, unless explicitly forbidden: the first element is always a function name; evaluating it means to call the function body; each of the rest elements will then be evaluated, and their values returned as the arguments for the function. Examples >(sqrt x) Error: The variable X is unbound. >(+ (sqrt 4) 4.0) 6.0 >(+ (/ 3 5) 4) 23/5

3) To assign a value to a symbol (setq, set, setf) setq is a special form of function (with two arguments); the first argument is a symbol which will not be evaluated; the second argument is a S-expression, which will be evaluated; the value of the second argument is assigned to be the value of the first argument to forbid evaluation of a symbol (quote or ) >(setq x 3.0) 3.0 >x 3.0 >(setq y x) 3.0 ; the value of x is assigned as the value of y >y 3.0 >(+ x y) 6.0

(quote x) x >'x x >(setq z 'x) x >(+ x z) Error: X is not of type NUMBER... >(+ x (eval z)) 6.0 eval">

to force an evaluation, using function "eval" Two more assignment functions: (set x y) ; assign the value of y to the value of x. x is evaluated ; first and whose value must be a symbol ; "setq" is a combination of "set" and "quote" (setf x y) ; similar to but more general than "setq" in that x can be ; something other than a symbol. >(quote x) x >'x x >(setq z 'x) x >(+ x z) Error: X is not of type NUMBER... >(+ x (eval z)) 6.0 eval

first rest function nesting

car cdr cadr caddr nthcdr butlast cons append

Compositions of car and cdr

length reverse last list

Activity Construct a list of four birds by evaluating several expressions with cons. Find out what happens when you cons a list onto itself. Replace the first element of the list of four birds with a fish. Replace the rest of that list with a list of other fish.

Basic expression evaluation

2) Predicates (a special function which returns NIL if the predicate is false, T or anything other than NIL, otherwise) =, >, =, (< x y) NIL >(= x y) T >(equal x y) NIL >(equal a (car L)) T >(atom L) NIL >(listp x) NIL >(listp L) T >(atom x) T >(numberp x) NIL >(atom (car L)) T >(numberp x) T >(symbolp x) T >(symbolp x) NIL predicates

Basic storage handling

>(null L) NIL >(null NIL) T >(null x) NIL 3) Set operations ( a list can be viewed as a set whose members are the top elements of the list) >(member 'b L) ; test if symbol b is a member (a top element) of L (B C) ; if yes, returns the sublist of L starting at the ; first occurrence of symbol b >(member b (cons 'b L)) (B A B C) >(member x L) NIL ; if no, returns NIL >(union L1 L2) ; returns the union of the two lists >(intersection L1 L2) ; returns the intersection of the two lists >(set-difference L1 L2) ; returns the difference of the two lists Set operations

Defining New Functions (defun name (parameter*) "Optional documentation string." body) Convention: you construct compound names with hyphens rather than underscores or inner caps. Thus, frob-widget is better Lisp style than either frob_widget or frobWidget. When a parameter list is a simple list of variable names, the parameters are called required parameters

Optional Parameters Place the symbol &optional followed by the names of the optional parameters. (defun foo (a b &optional c d) (list a b c d)) When the function is called, arguments are first bound to the required parameters. (foo 1 2) (1 2 NIL NIL) (foo 1 2 3) (1 2 3 NIL) (foo 1 2 3 4) (1 2 3 4)

Non-NIL default value Replace the parameter name with a list containing a name and an expression. The expression will be evaluated only if the caller doesn't pass enough arguments to provide a value for the optional parameter. (defun foo (a &optional (b 10)) (list a b)) (foo 1 2) (1 2) (foo 1) (1 10)

More flexibility: (defun make-rectangle (width &optional (height width))...)

Rest Parameters Functions need to take a variable number of arguments. E.g. (+), (+ 1), (+ 1 2), (+ 1 2 3), A catchall parameter after the symbol &rest. Any arguments remaining after values have been doled out to all the required and optional parameters are gathered up into a list that becomes the value of the &rest parameter. (defun format (stream string &rest values)...) (defun + (&rest numbers)...)

Keyword Parameters Suppose you have a function that takes four optional parameters. Now suppose that most of the places the function is called, the caller wants to provide a value for only one of the four parameters. After any required, &optional, and &rest parameters you include the symbol &key and then any number of keyword parameter specifiers.

(defun foo (&key a b c) (list a b c)) (foo) (NIL NIL NIL) (foo :a 1) (1 NIL NIL) (foo :b 1) (NIL 1 NIL) (foo :c 1) (NIL NIL 1) (foo :a 1 :c 3) (1 NIL 3) (foo :a 1 :b 2 :c 3) (1 2 3) (foo :a 1 :c 3 :b 2) (1 2 3)

(defun foo (&key (a 0) (b 0 b-supplied-p) (c (+ a b))) Mixing Different Parameter Types Whenever more than one flavor of parameter is used, they must be declared in order : first the names of the required parameters, then the optional parameters, then the rest parameter, and finally the keyword parameters.

(* i j) n) (return-from foo (list i j))))))">

Function Return Values The default behavior: return the value of the last expression evaluated as the functions own return value. The RETURN-FROM special operator to immediately return any value from the function The first "argument" is the name of the block from which to return. (defun foo (n) (dotimes (i 10) (dotimes (j 10) (when (> (* i j) n) (return-from foo (list i j))))))

Data structure s assoc

make-array aref defstruct

Dotted pairs

4) Conditional >(cond ( ). ( )) each ( ) is called a clause; if test-i (start with i=1) returns T (or anything other than NIL), this function returns the value of action-i; else, go to the next clause; usually, the last test is T, which always holds, meaning otherwise. cond can be nested (action-i may contain (cond...)) conditional

5. Define functions (heavy use of recursive definitions) (defun func-name (arg-1... Arg-n) func-body) examples: (defun member (x L) (cond ((null L) nil) ; base case 1: L is empty ((equal x (car L)) L) ; base case 2: x=first(L) (t (member x (cdr L))) ; recursion: test if x is in rest(L) )) (defun intersection (L1 L2) (cond ((null L1) nil) ((null L2) nil) ((member (car L1) L2) (cons (car L1) (intersection (cdr L1) L2))) (t (intersection (cdr L1) L2)) )) Example: (intersection '(a b c) '(b a b c)) returns (a b c) (intersection '(b a b c) '(a b c)) returns (b a b c) Now, having basic functions, defun and cond we can define any Lisp function. Examples. member intersection

(defun set-difference (L1 L2) (cond ((null L1) nil) ((null L2) L1) ((not (member (car L1) L2)) (cons (car L1) (set-difference (cdr L1) L2))) (t (set-difference (cdr L1) L2)) )) Define functions iteratively. (dolist (x L result) body) for each top level element x in L, do body(x); x is not equal to an element of L in each iteration, but rather x takes an element of L as its value; (dotimes (count n result) body) ; do body n times. count starts with 0, ends with n-1 Note: result is optional, to be used to hold the computing result. If result is given, the function will return the value of result, returns NIL, otherwise. (may change global variables as side effects.) dolist dotimes

Activity Write a function sum to calculate the sum of all numbers in a list

(defun sum1 (L) (setq y 0) (dolist (x L y) (setq y (+ y x)))) (defun sum2 (L) (setq y 0) (dolist (x L y) (setq y (+ y (eval x))))) (defun sum3 (L) (setq y 0) (dotimes (count (length L) y) (setq y (+ y (nth count L))) )) defun sum4 (L) (setq y 0) (dotimes (count (length L) y) (setq y (+ y (eval (nth count L)))) )) dotimes dolist Various definitions of SUM >(setq L1 '(1 2 3)) (1 2 3) >(sum1 L1) 6

>(setq L1 '(1 2 3)) (1 2 3) >(setq L2 '(a b c)) (A B C) >(dotimes (count 3) (set (nth count L2) (nth count L1))) NIL >a 1 >(sum1 L1) 6 >(sum3 L1) 6 >(sum1 L2) Error: >(sum3 L2) Error: >(sum2 L2) 6 >(sum4 L2) 6

comp 205 – week 11 dr. chunbo chu. intro lisp stands for “list process” invented by john...

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