Lecture 6-2CS250: Intro to AI/Lisp Programming in Your Favorite Language Lecture 5-2 February 11 th, 1999 CS250

Lecture 6-2CS250: Intro to AI/Lisp “Get Your Red-Hot Lists Here!” Conses are pairs of pointers –First pointer is the car –Rest is the cdr Lists are conses in which: –First pointer is the first element –Second pointer is the rest of the list –No intermediate pointers makes last expensive USER(104): (last (list 'a 'b 'c)) (C)

Lecture 6-2CS250: Intro to AI/Lisp Box & Pointer Represent a cons graphically (list ‘a (list ‘b ‘c) ‘d)

Lecture 6-2CS250: Intro to AI/Lisp Some Things are More Equal than Others Lisp has multiple definitions of equality Decreasing order of strictness –eq, eql, equal

Lecture 6-2CS250: Intro to AI/Lisp eq True if its arguments are the same, identical object; otherwise, returns false (eq 'a 'b) => false (eq 'a 'a) => true (eq 3 3) => true OR => false (eq 3 3.0) => false

Lecture 6-2CS250: Intro to AI/Lisp eql True of two objects, x and y, in the folowing cases: 1. If x and y are eq. 2. If x and y are both numbers of the same type and the same value. 3. If they are both characters that represent the same character. (eql 'a 'b) => false (eql 'a 'a) => true (eql 3 3) => true (eql 3 3.0) => false (eql ) => true (eql #c(3 -4) #c(3 -4)) => true (eql #c(3 -4.0) #c(3 -4)) => false

Lecture 6-2CS250: Intro to AI/Lisp equal Generally, returns true if two objects print the same > (setf x (cons ‘a nil)) (A) > (eql x x) T > (equal x (cons ‘a nil)) T

Lecture 6-2CS250: Intro to AI/Lisp Mapping over lists Need to do something to every element in a list? Try a mapping function: –mapcar for using the car of successive cdr’s –maplist for successive cdr’s themselves

Lecture 6-2CS250: Intro to AI/Lisp mapcar in Action USER(115): (mapcar #'list '(a b c) '( )) USER(116): (mapcar #'list '(a b c) '(1 2)) ((A 1) (B 2) (C 3)) ((A 1) (B 2))

Lecture 6-2CS250: Intro to AI/Lisp Creating an N-Queens Problem (defun nqueens-initial-state (n &optional (explicit? nil) (complete? nil)) (let ((s (make-CSP-state :unassigned (mapcar #'(lambda (var) (make-CSP-var :name var :domain (iota n))) (iota n)) :assigned nil :constraint-fn (if explicit? (let ((constraints (nqueens-constraints n))) #'(lambda (var1 val1 var2 val2) (CSP-explicit-check var1 val1 var2 val2 constraints))) #'nqueens-constraint-fn)))) (if complete? (CSP-random-completion s) s)))

Lecture 6-2CS250: Intro to AI/Lisp Unassigned Variables (mapcar #'(lambda (var) (make-CSP-var:name var :domain (iota n))) (iota n)) USER(105): (iota 8) ( )

Lecture 6-2CS250: Intro to AI/Lisp Unassigned Variables II ((0 ( ) NIL NIL) (1 ( ) NIL NIL) (2 ( ) NIL NIL) (3 ( ) NIL NIL) (4 ( ) NIL NIL) (5 ( ) NIL NIL) (6 ( ) NIL NIL) (7 ( ) NIL NIL))

Lecture 6-2CS250: Intro to AI/Lisp Recursion Again Recursive function = Base case + Recursive step –Base case will be a conditional test, plus a call that returns Example: General-Search (defun general-search-helper (problem nodes queuing-fn) (let ((node (first nodes))) (if (null node) nil :

Lecture 6-2CS250: Intro to AI/Lisp Recursive General Search (if (goal-test problem (node-state node)) node (general-search-helper problem (funcall queuing-fn (rest nodes) (expand node problem)) queuing-fn)...) If we’ve got a node, what do we do next? What if it’s not the goal?

Lecture 6-2CS250: Intro to AI/Lisp Put it Together (defun general-search-helper (problem nodes queuing-fn) (let ((node (first nodes))) (if (null node) nil (if (goal-test problem (node-state node)) node (general-search-helper problem (funcall queuing-fn (rest nodes) (expand node problem)) queuing-fn)))))

Lecture 6-2CS250: Intro to AI/Lisp Getting it Started... (let ((nodes (make-initial-queue problem queuing-fn)) node) (defun make-initial-queue (problem queuing-fn) (let ((q (make-empty-queue))) (funcall queuing-fn q (list (create-start-node problem))) q)) From simple.lisp : General-Search function How does Make-Initial-Queue work?

Lecture 6-2CS250: Intro to AI/Lisp Top-level Function (defun general-search-recursive (problem queueing-fn) "Recursive version of general search" (general-search-helper problem (list (create-start-node problem)) queueing-fn))

Lecture 6-2CS250: Intro to AI/Lisp BFS with a List (defun breadth-first-search (problem) (general-search-recursive problem #'append)) What’s the rule for node exploration in BFS? How are new nodes added?

Lecture 6-2CS250: Intro to AI/Lisp Sets Sets let you treat lists as sets –Membership –Union, intersection –Set difference

Lecture 6-2CS250: Intro to AI/Lisp Sequences Sequences include more than just lists –Ordered series –Lists and vectors Many functions operate on sequences, not just lists: –length, sort, subseq, reverse, every, some, elt

Lecture 6-2CS250: Intro to AI/Lisp Structures Create records in Lisp Define structures with the defstruct macro: (defstruct point x y)

Lecture 6-2CS250: Intro to AI/Lisp “Big Sale on Constructors & Accessors!” Creating a structure creates: –Constructor ( make-point ) Arguments are passed by keyword –Copy constructor ( copy-point ) –Slot accessor functions ( point-x, point-y ) –Type predicate ( point-p ) New structures are new types

Lecture 6-2CS250: Intro to AI/Lisp Default Values for Structure Fields Add a default value (defstruct midterm (difficulty (progn (format t “How hard was it?”) (read))) (max-grade 54) (num-completed nil))

Lecture 6-2CS250: Intro to AI/Lisp Customize Automatic Functions (defstruct (point (:conc-name p) (:print-function print-point)) (x 0) (y 0)) (defun print-point (p stream depth) (format stream “# ” (px p) (py p)))