Assignment:Changing Data

CMSC 11500

Introduction to Computer Programming

November 1, 2002

Roadmap

• Recap: Binary Search Trees

• State: Changing Data over Time– Assignment with set!– Multiple statements with begin– Objects with state

• Traffic light example• Bank account example

• Summary

Sets: Binary Search Trees• Bst is:

• 1) #f• 2) (make-bt val left right)

– , where val: number; left, right: bst

• (define-struct bt (val left right))• INVARIANT: For node n, all vals in (bt-left n) < n; all vals in

(bt-right n) > n

• Define operations on sets with bst’s– Element-of?, Adjoin, Intersection, Union,..– More efficient – O(log n) – search, adjunction..– Functions exploit & maintain invariant

• If true of inputs, must be kept true of output

State of the Data

• Computationally model objects in world

• Objects change over time– E.g. traffic light: red, yellow, green– Bank account: balance up/down– Age, temperature, etc,…

Changing State in Scheme

• So far, value in variable once– Bind values to variables with formal params

• To change state must change value assigned to variable

• Assignment: set!– (set! var exp)

• var: variable name; exp: any scheme expression

– (set! x 1)– (set! x (factorial 10))

Evaluating set!

• Expressions evaluate to values– (+ 3 4) => 7– (set! x 1) ??? Void - nothing

• Set! is produces an effect, not a value– Changes value of variable

• Problem: Need to set! AND return value• Solution: Do both!

– (begin exp1 …. expn)– Does evaluates all exps; returns value of last– (begin (set! x (+ 3 4)) x) -> 7

Example: Traffic Lights

• State of traffic light: color– TL-color is ‘green, ‘yellow, or ‘red– ; contract: current-color: TL-color

• (define current-color ‘red)– ? (set! current-color 5)– ? (set! current-color ‘green)

Changing Traffic Lights

• First step: What would it change to?

• Function: – ;;contract:next-color: TL-color -> TL-color– ;; purpose: To identify next color in sequence

(define (next-color color) (cond ((eq? color ‘red) ‘green)

((eq? color ‘yellow) ‘red) ((eq? color ‘green) ‘yellow)))

Really Changing Lights

• Key: change value of state variable with set!

(define (turn-light) (begin (set! current-color (next-color current-color)) current-color))) (turn-light)

Return value?Current-color?

Example: Bank Account

• State of bank account: balance– Varies over time: withdrawal, deposit, check..

• (define balance 100)

• (set! balance (- balance 25))

• Goal: encapsulation in function– Test if enough money

• If there is, reduce balance• O.w., report error

Make-Withdraw

• Contract: – ;;make-withdraw: number ->

(number -> number)– Note: returns a procedure

• Purpose:– ;; to create a procedure to process withdrawal

Make-withdraw

(define (make-withdraw initial-balance) (let ((balance initial-balance)) (lambda (amount)

(if (<= amount balance) (begin

(set! balance (- balance amount))balance)

(error “Insufficient funds!!))))

Using Make-withdraw

• (define w1 (make-withdraw 100))

• (w1 20)– > 80

• (w1 100)– > “Insufficient funds!!”

• (w1 60)– > 20

Summary

• State variables– Represent objects: state changes over time

• Forms:– (set! var expression)– (begin exp1 … expn)

• Modeling traffic lights and bank accounts