Intelligent Agents

Intelligent Agents Lecture 3-2 October 14th, 1999 CS250 CS250: Intro to AI/Lisp

Announcements • No class on Thursday (October 7th) • Next week • Tuesday, Thursday in class • Wednesday 7-9pm in Maclab CS250: Intro to AI/Lisp

Assignment 1 • Turing’s comments • Skimpy answers • Most support: Continuity of nervous system, consciousness • Undecidability and intractability • Following instructions • Genes = programs • Lisp code CS250: Intro to AI/Lisp

Answering Lisp Questions • Style counts • setf is evil • Think it works? Try it out! • Grab your functions by the tail CS250: Intro to AI/Lisp

Intelligent Agents • What’s an agent? • Little people in the TV CS250: Intro to AI/Lisp

(defstructure environment "The world in which agents exist.” ;; A list of the agents in the environment (agents '()) ;; The number of time steps simulated so far (step 0) ;; Stop the simulation after this number (max-steps 1000) ;; Stream to display output on (stream t) ;; Have we run initialize on this environment yet? (initialized nil) ;; Current state of the environment; other subtypes ;; add new slots to hold various state information (state nil) ) CS250: Intro to AI/Lisp

(defstructure agent "Agents take actions (based on percepts and the agent program) and receive a score (based on the performance measure). An agent has a body which can take action, and a program to choose the actions, based on percepts." (program #'nothing); fn: percept -> action (body (make-agent-body)) (score 0) (percept nil) (action nil) (name nil)) CS250: Intro to AI/Lisp

(defun run-environment (env) "Basic environment simulator. It gives each agent its percept, gets an action from each agent, and updates the environment. It also keeps score for each agent, and optionally displays intermediate results. [p 48]" (initialize env) (display-environment env) (dotimes (i (environment-max-steps env)) (incf (environment-step env)) ;; Deliver percept and get action from each agent (for each agent in (environment-agents env) do (setf (agent-percept agent) (get-percept env agent)) (setf (agent-action agent) (funcall (agent-program agent) (agent-percept agent)))) ;; Execute the actions and otherwise update the world (update-fn env) ;; Update the agent scores, then optionally display the current state (for each agent in (environment-agents env) do (setf (agent-score agent) (performance-measure env agent))) (display-environment env) (when (termination? env) (RETURN))) env) CS250: 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) CS250: Intro to AI/Lisp

nil d a nil b c Box & Pointer • Represent a cons graphically (list ‘a (list ‘b ‘c) ‘d) CS250: Intro to AI/Lisp

Some Things are More Equal than Others • Lisp has multiple definitions of equality • Decreasing order of strictness • eq, eql, equal CS250: 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 CS250: 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 3.0 3.0) => true (eql #c(3 -4) #c(3 -4)) => true (eql #c(3 -4.0) #c(3 -4)) => false CS250: 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 CS250: 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 CS250: Intro to AI/Lisp

mapcar in Action USER(115): (mapcar #'list '(a b c) '(1 2 3 4)) ((A 1) (B 2) (C 3)) USER(116): (mapcar #'list '(a b c) '(1 2)) ((A 1) (B 2)) CS250: 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))) CS250: Intro to AI/Lisp

Unassigned Variables (mapcar #'(lambda (var) (make-CSP-var :name var :domain (iota n))) (iota n)) USER(105): (iota 8) (0 1 2 3 4 5 6 7) CS250: Intro to AI/Lisp

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

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

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

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

Sets • Sets let you treat lists as sets • Membership • Union, intersection • Set difference CS250: 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 CS250: Intro to AI/Lisp

Structures • Create records in Lisp • Define structures with the defstruct macro: (defstruct point x y) CS250: 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 CS250: 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)) CS250: 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 “#<~A, ~A>” (px p) (py p))) CS250: Intro to AI/Lisp

Intelligent Agents