Notes: Wednesday 27 February 2002

Schedule

• Now: Exam 1
• Thursday, 28 February, 3:30 in Wilson Hall Room 402: Steven Pinker (MIT Department of Brain and Cognitive Sciences), The Blank Slate, the Noble Savage, and the Ghost in the Machine
• Friday, 8 March: Problem Set 5
• 9-17 March: Spring Break
• Before 18 March: GEB all of Part I

Code: countletters.ss

Notes

Mutation Procedures:

• (set! Name Expression) — evaluate Expression and replace the value associated with Name with the value of Expression
• (set-car! Expression₀ Expression₁) — evaluate Expression₀ and Expression₁ and store the value of Expression₁ in the car part of the value of Expression₀. Expression₀ must evaluate to a cons pair.
• (set-cdr! Expression₀ Expression₁) — evaluate Expression₀ and Expression₁ and store the value of Expression₁ in the cdr part of the value of Expression₀. Expression₀ must evaluate to a cons pair.

Why is mutation scary?

Why is mutation useful?

Why do our evaluation rules fail when we allow mutation?

Would it be possible to create a circular list without using set-cdr!?

How would you count letters without using mutation?

(define (make-letter-tallys)
  (for (char->integer #\a) 
       (char->integer #\z) 
       (lambda (accum cno) 
         (append accum (list (cons (integer->char cno) 0))))
       null))
                                           
(define (count-letters msg)
  (let ((letter-uses (make-letter-tallys)))
    (map (lambda (c) 
           (insertlg 
            (lambda (thisletter foundit)
              (if foundit foundit 
                  (if (eq? (car thisletter) c) 
                      (begin (set-cdr! thisletter (+ (cdr thisletter) 1)) #t)
                      #f)))          
            letter-uses
            #f))
         (string->list msg))
    letter-uses))

University of Virginia Department of Computer Science CS 200: Computer Science

David Evans evans@virginia.edu Using these Materials