Lecture 15: Tables and OOP

Last Lecture: Stack Data Abstraction Constructor: (make-stack) returns an empty stack Selector: (top stack) returns current top element from a stack Mutators and operations: (insert stack elem) returns a new stack with the element added to the top of the stack (delete stack) returns a new stack with the top element removed from the stack (empty-stack? stack) returns #t if no elements, #f otherwise

Stack Data Abstraction Insert Insert First in, Last out. Insert Delete Insert

Queue Data Abstraction Constructor: (make-queue) returns an empty queue Selector: (front-queue q) returns the object at the front of the queue. If queue is empty signals error Mutators and operations: (insert-queue q elt) returns a new queue with elt at the rear of the queue (delete-queue q) returns a new queue with the item at the front of the queue removed (empty-queue? q) tests if the queue is empty

Queue illustration Insert Insert First in, First out Insert x1 x2 x3 front

Queue illustration Insert Insert First in, First out Insert Delete x2 front

Queue Contract If q is a queue, created by (make-queue) and subsequent queue procedures, where i is the number of insertions, j is the number of deletions, and xi is the ith item inserted into q , then If j>i then it is an error If j=i then (empty-queue? q) is true, and (front-queue q) and (delete-queue q) are errors. If j<i then (front-queue q) = xj+1

Today: Tables -- get and put

One dimentional tables c d b a 1 2 3 4 (define (lookup key table) (let ((record (assoc key (cdr table)))) (if record (cdr record) false)))

One dimentional tables (define (lookup key table) (let ((record (assoc key (cdr table)))) (if record (cdr record) false))) (define (assoc key records) (cond ((null? records) false) ((equal? key (caar records)) (car records)) (else (assoc key (cdr records)))))

One dimentional tables (define (insert! key value table) (let ((record (assoc key (cdr table)))) (if record (set-cdr! record value) (set-cdr! table (cons (cons key value) (cdr table))))) 'ok) Example: (insert! ‘e 5 table)

One dimentional tables (define (insert! key value table) (let ((record (assoc key (cdr table)))) (if record (set-cdr! record value) (set-cdr! table (cons (cons key value) (cdr table))))) 'ok) *table* c d b a 1 2 3 4 e 5

One dimentional tables (define (make-table)(list '*table*)) *table*

Two dimentional tables presidents Bush 88 Clinton 92 elections Florida Bush Bush NY Gore California

Two dimentional tables (define (lookup key-1 key-2 table) (let ((subtable (assoc key-1 (cdr table)))) (if subtable (let ((record (assoc key-2 (cdr subtable)))) (if record (cdr record) false)) false))) Example: (lookup ‘elections ‘Florida table) ==> Bush

Two dimentional tables (define (insert! key-1 key-2 value table) (let ((subtable (assoc key-1 (cdr table)))) (if subtable (let ((record (assoc key-2 (cdr subtable)))) (if record (set-cdr! record value) (set-cdr! subtable (cons (cons key-2 value) (cdr subtable))))) (set-cdr! table (cons (list key-1 (cons key-2 value)) (cdr table))))) 'ok) Example: (insert! ‘elections ‘California ‘Gore table) ==> okbb

Two dimentional tables presidents Bush 88 Clinton 92 elections Florida Bush Bush Gore NY Gore California

Two dimentional tables Example: (insert! ‘singers ‘Madona ‘M table) ==> ok

Two dimentional tables presidents *table* Bush 88 Clinton 92 singers elections NY Florida Gore Bush California Madona M

Implement get and put (define oper-table (make-table)) (define (put x y v) (insert! x y v oper-table)) (define (get x y) (lookup x y oper-table))

Introduction to Object Oriented Programming

One View of Data "Generic" operations by looking at types: Tagged data: Some complex structure constructed from cons cells Explicit tags to keep track of data types Implement a data abstraction as set of procedures that operate on the data "Generic" operations by looking at types: (define (real-part z) (cond ((rectangular? z) (real-part-rectangular (contents z))) ((polar? z) (real-part-polar (contents z))) (else (error "Unknown type -- REAL-PART" z))))

An Alternative View of Data: Procedures with State A procedure has parameters and body as specified by l expression environment (which can hold name-value bindings!) Can use procedure to encapsulate (and hide) data, and provide controlled access to that data constructor, accessors, mutators, predicates, operations mutation: changes in the private state of the procedure

Example: Pair as a Procedure with State (define (cons x y) (lambda (msg) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) (else (error "pair cannot" msg))))) (define (car p) (p ‘CAR)) (define (cdr p) (p ‘CDR)) (define (pair? p) (and (procedure? p) (p ‘PAIR?)))

Reminder: What is our "pair" object? 1 p: msg body: (cond ...) E1 foo: x: 1 y: 2 (define foo (cons 1 2)) 2 3 (car foo) becomes (foo 'CAR) GE p: x y body: (l (msg) (cond ..)) cons: (car foo) | GE => (foo 'CAR) | E2 => 2 (cond ...) | E3 => x | E3 => 1 msg: CAR E3 3

Pair Mutation as Change in State (define (cons x y) (lambda (msg) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) ((eq? msg ‘SET-CAR!) (lambda (new-car) (set! x new-car))) ((eq? msg ‘SET-CDR!) (lambda (new-cdr) (set! y new-cdr))) (else (error "pair cannot" msg))))) (define (set-car! p new-car) ((p ‘SET-CAR!) new-car)) (define (set-cdr! p new-cdr) ((p ‘SET-CDR!) new-cdr))

Example: Mutating a pair object 1 p: msg body: (cond ...) E4 bar: x: 3 y: 4 (define bar (cons 3 4)) (set-car! bar 0) | GE => ((bar 'SET-CAR!) 0) | E5 2 (set-car! bar 0) GE 6 changes x value to 0 in E4 (cond ...) | E6 => (l (new-car) (set! x new-car)) | E6 msg: SET-CAR! E6 3 (set! x new-car) | E7 new-car: 0 E7 5 p: new-car body: (set! x new-car) 4

Message Passing Style - Refinements lexical scoping for private state and private procedures (define (cons x y) (define (change-car new-car) (set! x new-car)) (define (change-cdr new-cdr) (set! y new-cdr)) (lambda (msg . args) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) ((eq? msg ‘SET-CAR!) (change-car (car args))) ((eq? msg ‘SET-CDR!) (change-cdr (car args))) (else (error "pair cannot" msg))))) (define (car p) (p 'CAR)) (define (set-car! p val) (p 'SET-CAR! val))

Message Passing Style - Refinements lexical scoping for private state and private procedures (define (cons x y) (define (change-car new-car) (set! x new-car)) (define (change-cdr new-cdr) (set! y new-cdr)) (lambda (msg . args) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) ((eq? msg ‘SET-CAR!) (change-car (car args))) ((eq? msg ‘SET-CDR!) (change-cdr (car args))) (else (error "pair cannot" msg))))) (define (car p) (p 'CAR)) (define (set-car! p val) (p 'SET-CAR! val))

Programming Styles – Procedural vs. Object-Oriented Procedural programming: Organize system around procedures that operate on data (do-something <data> <arg> ...) (do-another-thing <data>) Object-based programming: Organize system around objects that receive messages (<object> 'do-something <arg>) (<object> 'do-another-thing) An object encapsulates data and operations Message passing and procedure are the means to write Object Oriented code in scheme

Tables in OO style (define (make-table) (let ((local-table (list '*table*))) (define (lookup key-1 key-2) . . . ) (define (insert! key-1 key-2 value) 'ok) (define (dispatch m) (cond ((eq? m 'lookup-proc) lookup) ((eq? m 'insert-proc!) insert!) (else (error "Unknown operation -- TABLE" m)))) dispatch))

Table in OO style (define operation-table (make-table)) (define get (operation-table 'lookup-proc)) (define put (operation-table 'insert-proc!))

(define oper-table (make-table)) | GE local-table *table* p: b: (let ((local-table (list '*table*))) . . . ) lookup: p: key-1 key-2 b: . . . insert!: dispatch:

Object-Oriented Programming Terminology Class: specifies the common behavior of entities in scheme, a "maker" procedure E.g. cons or make-table in our previous examples Instance: A particular object or entity of a given class in scheme, an instance is a message-handling procedure made by the maker procedure E.g. foo or bar or oper-table in our previous examples

Stacks in OO style (define (make-stack) (let ((top-ptr '())) (define (empty?) (null? top-ptr)) (define (delete!) (if (null? top-ptr) (error . . .) (set! top-ptr (cdr top-ptr))) top-ptr ) (define (insert! elmt) (set! top-ptr (cons elmt top-ptr)) top-ptr) (define (top) (if (null? top-ptr) (error . . .) (car top-ptr))) (define (dispatch op) (cond ((eq? op 'empty?) empty?) ((eq? op 'top) top) ((eq? op 'insert!) insert!) ((eq? op 'delete!) delete!))) dispatch))

Stacks in OO style (define s (make-stack)) ==> ((s 'insert!) 'a) ==> ((s 'insert!) 'b) ==> ((s 'top)) ==> ((s 'delete!)) ==> undef (a) (b a) b (a) a ()

Queues in OO style A lazy approach: We know how to do stacks so lets do queues with stacks :) We need two stacks: stack2 stack1 delete insert

Queues in OO style a a b a b b c b c ((q ‘insert) ‘a) ((q ‘insert) ‘b) ((q ‘delete)) b b c ((q ‘insert) ‘c) ((q ‘delete)) c

Queues in OO style (define (make-queue) (let ((stack1 (make-stack)) (stack2 (make-stack))) (define (reverse-stack s1 s2) _______________) (define (empty?) (and ((stack1 'empty?)) ((stack2 'empty?)))) (define (delete!) (if ((stack2 'empty?)) (reverse-stack stack1 stack2)) (if ((stack2 'empty?)) (error . . .) ((stack2 'delete!)))) (define (first) ((stack2 'top)))) (define (dispatch op) (cond ((eq? op 'empty?) empty?) ((eq? op 'first) first) ((eq? op 'delete!) delete!) (else (stack1 op)))) dispatch))

Queues in OO style Inheritance: One class is a refinement of another The queue class is a subclass of the stack class