CS 3015 Lecture 16

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CS 3015 Lecture 16
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Evaluating set!

• Evaluating the expression
• (set! ltvariablegt ltvaluegt)
• Locates the binding of the variable in the
  environment and changes that binding to the new
  value

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Return to an Early Example

• (define (square x)( x x))
• (define (sum-of-squares x y)( (square x) (square
   y)))
• (define (f a)(sum-of-squares ( a 1) ( a 2)))

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Example contd
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Evaluating (f 5)
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Comments

• Observe that each call to square creates a new
  environment containing a binding for x
• different frames serve to keep separate the
  different local variables all named x
• Each frame created by square points to the global
  environment, since this is the environment
  indicated by the square procedure object.

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Frames as a Repository for Local State

• Frames are used to create instance variables
• (define (make-withdraw balance)(lambda (amount) 
   (if (gt balance amount)      (begin 
• (set! balance (- balance amount))       bal
  ance)    "Insufficient funds")))

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The Withdraw Example

• Let us describe the evaluation of
• (define W1 (make-withdraw 100))
• followed by
• (W1 50)50

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Result of Defining Make-withdraw
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Evaluating (define W1 (make-withdraw 100))
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Evaluating (W1 50)
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Evaluating (define W2 (make-withdraw 100))
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Internal Definitions

• (define (sqrt x)(define (good-enough? guess)  (lt
   (abs (- (square guess) x))
•  0.001))(define (improve guess)  (average 
  guess (/ x guess)))(define (sqrt-iter guess)  (i
  f (good-enough? guess)      guess      (sqrt-ite
  r (improve guess))))(sqrt-iter 1.0))

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Why Internal Definitions Behave as Desired
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Comments

• The names of the local procedures do not
  interfere with external names, because the local
  names are bound in the frame created when the
  procedure is run
• The local procedures can access the arguments of
  the enclosing procedure, simply by using
  parameter names as free variables.
• The local procedure is evaluated in an
  environment that is subordinate to the evaluation
  environment for the enclosing procedure.

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Example Environment Model

• (define (product f a next b)
• (if (gt a b) 1
• ( (f a)
• (product f (next a) next b))))
• (define (fact n)
• (product (lambda (x) x) 1
• (lambda (x) ( x 1)) n))

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Global Env
Product fact
Arguments n Code
Arguments f,a,next,b Code
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Global Env
Product fact
(fact 3)
Arguments n Code
Arguments f,a,next,b Code
E1
n 3
Call to fact
f
a 1 next b 3
Arguments x Code x
Arguments x Code x1
First call to product
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Global Env
Product fact
(fact 3)
Arguments n Code
Arguments f,a,next,b Code
E1
n 3
Call to fact
f
E3
a 1 next b 3
f
a 2 next b 3
Arguments x Code x
Arguments x Code x1
E2
First call to product
Second call to product
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Modeling with Mutable Data

• We now need to return to our design methodology
  and see how it should be changed given mutable
  data
• In addition to selectors and constructors, we
  will now have mutators
• (set-balance ltaccountgt ltnew-valuegt)
• We start with mutators for the built in data
  object cons cells
• Up to now, we have not been able to modify lists

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Mutable List Structure

• Introduce two new operations
• set-car! and set-cdr!
• (set-car! x y)

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Effect of set-cdr!

• (define z (cons y (cdr x)))

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(set-cdr! x y)
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Could Implement cons as

• (define (cons x y)
• (let ((new (get-new-pair)))
• (set-car! new x)
• (set-cdr! new y)
• new))

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Sharing and Identity

• (define x (list 'a 'b))(define z1 (cons x x))

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In contrast to

• The list z2 formed by
• (cons (list 'a 'b) (list 'a 'b))

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Consider the Difference

• (define (set-to-wow! x)  (set-car! (car x) 'wow)
    x)
• Z1
• ((a b) a b)
• (set-to-wow! z1)
• ((wow b) wow b)
• Z2
• ((a b) a b)
• (set-to-wow! z2)
• ((wow b) a b)

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Generalization of eq?

• In addition to testing for same symbol, eq? tests
  whether or not two pointers are the same
• (eq? (car Z1) (cdr Z1))
• true
• (eq? (car Z2) (cdr Z2))
• ()

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Mutation is Just Assignment

• (define (cons x y)  (define (set-x! v) (set! x v)
  )  (define (set-y! v) (set! y v))  (define (disp
  atch m)    (cond ((eq? m 'car) x)          ((eq?
   m 'cdr) y)          ((eq? m 'set-car!) set-x!) 
           ((eq? m 'set-cdr!) set-y!)          (els
  e (error " m))))  dispatch)

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• (define (car z) (z 'car))
• (define (cdr z) (z 'cdr))
• (define (set-car! z new-value)  ((z 'set-car!) ne
  w-value)  z)
• (define (set-cdr! z new-value)  ((z 'set-cdr!) ne
  w-value)  z)