Lecture 2-1CS250: Intro to AI/Lisp Intelligent Agents Lecture 3-2 October 14 th, 1999 CS250.

Lecture 2-1 CS250: Intro to AI/Lisp

Intelligent Agents

Lecture 3-2

October 14th, 1999

CS250


Announcements

• No class on Thursday (October 7th)

• Next week– Tuesday, Thursday in class– Wednesday 7-9pm in Maclab


Assignment 1

• Turing’s comments– Skimpy answers– Most support: Continuity of nervous

system, consciousness

• Undecidability and intractability

• Following instructions– Genes = programs

• Lisp code


Answering Lisp Questions

• Style counts– setf is evil

• Think it works? Try it out!

• Grab your functions by the tail


Intelligent Agents

• What’s an agent?– Little people in the TV


(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))


(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))


(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)


“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)


Box & Pointer• Represent a cons graphically

(list ‘a (list ‘b ‘c) ‘d)


Some Things are More Equal than Others

• Lisp has multiple definitions of equality

• Decreasing order of strictness– eq, eql, equal


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


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


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


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


mapcar in Action

USER(115): (mapcar #'list '(a b c) '(1 2 3 4))

USER(116): (mapcar #'list '(a b c) '(1 2))

((A 1) (B 2) (C 3))

((A 1) (B 2))


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)))


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)


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))


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

:


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?


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)))))


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?


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))


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?


Sets

• Sets let you treat lists as sets– Membership– Union, intersection– Set difference


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


Structures

• Create records in Lisp

• Define structures with the defstruct macro:

(defstruct point

x

y)


“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


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))


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)))

Lecture 2-1CS250: Intro to AI/Lisp Intelligent Agents Lecture 3-2 October 14 th, 1999 CS250.

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Transcript of Lecture 2-1CS250: Intro to AI/Lisp Intelligent Agents Lecture 3-2 October 14 th, 1999 CS250.