1. David Evans http://www.cs.virginia.edu/evans
Lecture 17:
Environments
CS200: Computer Science
University of Virginia
Computer Science
David Evans

2. Menu Environments Evaluation Rules Exam 1 26 February 2003
CS 200 Spring 2003

3. Environments
26 February 2003
CS 200 Spring 2003

4. (define nest (lambda (x) (+ x x)))) > ((nest 3) 4) 8
Does the substitution model of evaluation tell us how to evaluate
this?
26 February 2003
CS 200 Spring 2003

5. Review: Names, Places, Mutation
A name is a place for storing a value.
define creates a new place
cons creates two new places, the car and the cdr
(set! name expr) changes the value in the place name to the value of expr
(set-car! pair expr) changes the value in the car place of pair to the value of expr
(set-cdr! pair expr) changes the value in the cdr place of pair to the value of expr
26 February 2003
CS 200 Spring 2003

6. Lambda and Places (lambda (x) …) also creates a new place named x
The passed argument is put in that place
> (define x 3)
> ((lambda (x) x) 4) 4
> x 3
x : 3
x : 4
How are these
places different?
26 February 2003
CS 200 Spring 2003

7. Location, Location, Location
Places live in frames
An environment is a pointer to a frame
We start in the global environment
Application creates a new frame
All frames except the global frame have one parent frame
26 February 2003
CS 200 Spring 2003

8. Environments > (define x 3)
+ : #<primitive:+>
global
environment
null? : #<primitive:null?>
x : 3
The global environment points to the outermost frame. It starts with all Scheme primitives.
> (define x 3)
26 February 2003
CS 200 Spring 2003

9. Procedures > (define double (lambda (x) (+ x x)))
+ : #<primitive:+>
global
environment
null? : #<primitive:null?>
x : 3
double: ??
> (define double (lambda (x) (+ x x)))
26 February 2003
CS 200 Spring 2003

10. How to Draw a Procedure A procedure needs both code and an environment
We’ll see why soon
We draw procedures like this:
Environment pointer
Code pointer
parameters: x body: (+ x x)
26 February 2003
CS 200 Spring 2003

11. Procedures > (define double (lambda (x) (+ x x)))
+ : #<primitive:+>
global
environment
null? : #<primitive:null?>
x : 3
double:
> (define double
(lambda (x) (+ x x)))
parameters: x body: (+ x x)
26 February 2003
CS 200 Spring 2003

12. Application Old rule: (Substitution model)
Apply Rule 2: Compounds. If the procedure is a compound procedure, evaluate the body of the procedure with each formal parameter replaced by the corresponding actual argument expression value.
26 February 2003
CS 200 Spring 2003

13. Application
Construct a new frame, enclosed in the environment of this procedure
Make places in that frame with the names of each parameter
Put the values of the parameters in those places
Evaluate the body in the new environment
26 February 2003
CS 200 Spring 2003

14. global
environment
Construct a new frame, enclosed in the environment of this procedure
Make places in that frame with the names of each parameter
Put the values of the parameters in those places
Evaluate the body in the new environment
+ : #<primitive:+>
double:
x : 3
parameters: x body: (+ x x)
x : 4
> (double 4) 8
(+ 4 4)
(+ x x) 8
26 February 2003
CS 200 Spring 2003

15. (nest 3) (define nest (lambda (x) (+ x x)))) > ((nest 3) 4)
global
environment
(define nest
(lambda (x)
(+ x x))))
> ((nest 3) 4)
+ : #<primitive:+>
nest:
x : 3
parameters: x body:
(lambda (x) (+ x x))
x : 3
(nest 3)
((lambda (x) (+ x x)) 4)
x : 4
26 February 2003
CS 200 Spring 2003
(+ x x)

16. Evaluation Rule 2 (Names)
If the expression is a name, it evaluates to the value associated with that name.
To find the value associated with a name, look for the name in the frame pointed to by the evaluation environment. If it contains a place with that name, use the value in that place. If it doesn’t, evaluate the name using the frame’s parent environment as the new evaluation environment. If the frame has no parent, error (name is not a place).
26 February 2003
CS 200 Spring 2003

17. evaluate-name (define (evaluate-name name env)
(if (null? env) (error “Undefined name: …”)
(if (frame-contains name (get-frame env))
(lookup name (get-frame env))
(evaluate-name name
(parent-environment
(get-frame env))))))
Hmm…maybe we can define a Scheme interpreter in Scheme!
26 February 2003
CS 200 Spring 2003

18. How does CS200 compare to other CS classes?
26 February 2003
CS 200 Spring 2003

19. CS 101 (Spring 2001) 7. Why do programmers comment?
(a) Give compiler extra instructions
(b) Aid readability
(c) To hide code for debugging purposes
(d) (b) and (c)
(e) none of the above
“Correct” answer
26 February 2003
CS 200 Spring 2003

20. Why Comment? To assign credit, blame or (non) liability
/* I was drunk when I wrote this. */
To document external assumptions (e.g., this parameter must be a sorted list)
To remind you what you are doing (; This is a bug!)
To keep code readers entertained with jokes, poems, and stories
To make sure your employer has to rehire you to fix the code (comments in foreign languages are especially effective)
Because you are taking a course where programs are judged based on the number of comments in it, not the clarity and economy of your code
26 February 2003
CS 200 Spring 2003

21. CS 201: Software Development Methods (Fall 2001)
5. [13 points total] Recursion.
The “SuperCool” function is defined for positive integers greater than or equal to zero as:
supercool(i) = supercool(i-1) * supercool(i-2);
supercool(0) = 1;
supercool(1) = 2;
For example:
supercool(2) = 2 * 1 = 2
supercool(3) = 2 * 2 = 4
supercool(4) = 4 * 2 = 8
supercool(5) = 8 * 4 = 32
Write a recursive function that calculates supercool. Your function must be recursive to receive any credit.
26 February 2003
CS 200 Spring 2003

22. CS 415: Programming Languages (Spring 2002)
3. Define a function (zip op x y) that takes lists (x1 x2 x3 ....) and (y1 y2 y3 ...) and evaluated to the list ((op x1 y1) (op x2 y2) (op x3 y3) ...).
Examples:
> (zip + '( ) '( ))
( )
> (zip list '(a b c d) '( ))
((a 6) (b 7) (c 8) (d 9))
26 February 2003
CS 200 Spring 2003

23. zip (define zip map) (define (zip f list1 list2)
(if (null? list1) null
(cons (f (car list1) (car list2))
(zip f (cdr list1) (cdr list2)))))
(define zip map)
(Easy way)
26 February 2003
CS 200 Spring 2003

24. CS588: Cryptology QPIV AUH DKV UA PKEXHE QA ATHU QPKU QPH AQPHE AUH
Problem Set 1, question 1:
QPIV AUH DKV UA PKEXHE QA ATHU QPKU QPH AQPHE AUH
- DIVH CZO
THIS ONE WAS NO HARDER TO OPEN THAN THE OTHER ONE
- WISE GUY
(From Safecracker Meets Safecracker in “Surely You're Joking, Mr. Feynman!: Adventures of a Curious Character”, Richard P. Feynman.)
26 February 2003
CS 200 Spring 2003

25. CS588: Cryptology
Midterm, last question:
The following statements were taken from Microsoft's recently released "Safe Internet: Microsoft Privacy & Security Fundamentals" web site and their "The Ten Immutable Laws of Security" page). For each of these statements, indicate whether the statement is true, misleading or an outright falsehood. Support your answer with 1-3 sentences. An incorrect, but well-supported answer is worth more credit that a correct, poorly-supported answer. Some parts of the statements are bold to focus your attention.
a. (7) Unless your application provides encryption features such as the use of digital signatures, your messages are about as private as a letter sent in an unsealed envelope.
26 February 2003
CS 200 Spring 2003

26. CS588: Cryptology
Midterm, last question:
Outright Falsehood. Digital signatures (like physicial signatures) provide authenticity, not privacy. Anyone can read a signed message by decrypting using the signer's public key.
26 February 2003
CS 200 Spring 2003

27. CS 655 (Spring 2001)
1. Mergering
a. Write a Scheme procedure listadder that combines two lists by adding their elements. For example, (listadder '(3 1 3) '(3 4 2)) should produce (6 5 5).
26 February 2003
CS 200 Spring 2003

28. listadder (define (listadder list1 list2) (if (null? list1) '()
(cons (+ (car list1) (car list2))
(listadder (cdr list1)
(cdr list2)))))
26 February 2003
CS 200 Spring 2003

29. PhD Qualifying Exam
Write a procedure posfilter that takes a list of integers as input and evaluates to the list with all the non-positive elements removed. For example, (posfilter (list )) evaluates to ( )
(define posfilter enronize)
26 February 2003
CS 200 Spring 2003

30. If you don’t already have enronize…
(define (posfilter lst)
(if (null? lst)
null
(if (> (car lst) 0)
(cons (car lst) (posfilter (cdr lst)))
(posfilter (cdr lst)))))
If you got a 7 or better on question 5, you did better than more than half of our PhD students!
(18 of you got 10/10)
26 February 2003
CS 200 Spring 2003

31. Part d (of that question)
Write a procedure filter that takes a list of integers and a filter function as input and evaluates to the list with only the elements for which the filter function evaluates to true.
(define (filter lst f)
(if (null? lst)
null
(if (f (car lst))
(filter (cdr lst))
(cons (car lst) (filter (cdr lst))))))
26 February 2003
CS 200 Spring 2003

32. make-enronize
Why couldn’t I ask the PhD students to implement make-enronize?
26 February 2003
CS 200 Spring 2003

33. PhD Qualifying Exam
Define a procedure that takes a binary tree as input, and produces as output the mirror image of that tree, swapping the left and right sides at every node.
How much work is your procedure?
12 of you did better on this than our
best PhD student this year did!
26 February 2003
CS 200 Spring 2003

34. Charge No class Friday – enjoy your “break”!
But that doesn’t mean you have no work to do…
Mutation makes evaluation rules more complicated
Environment diagrams quickly get complicated – but like substitution evaluations, just follow rules mechanically
Don’t wait until after Spring Break to start PS5
Finish GEB part I before next class.
26 February 2003
CS 200 Spring 2003