CSC 153 Grinnell College Spring, 2005
 
Computer Science Fundamentals
Laboratory Exercise
 

Sorting and Mutation

Goals

This laboratory exercise provides experience with assignment as a mechanism to change a binding as a side effect of a procedure.

Environments

  1. Consider the following definition sequence:

    > (define str "original binding")
    > (let ((str "second binding"))
        (display "2: ") (display str) (newline)
        (let ((str "third binding"))
          (display "3: ") (display str) (newline)
          (let ((str "fourth binding"))
            (display "4: ") (display str) (newline)
            (let ((str "fifth binding"))
              (display "5: ") (display str) (newline))
            (display "4: ") (display str) (newline))
          (display "3: ") (display str) (newline))
        (display "2: ") (display str) (newline))
    

    Run this code, describe the output, and explain how that result is achieved.

  2. Consider the following interaction that appears in the reading:

    > (define ch #\A)
    > ch
    #\A
    
    > (define ch #\B)
    > ch
    #\B
    
    > (set! ch #\C)
    > ch
    #\C
    
    > (set! ch #\D)
    > ch
    #\D
    
    > (let ((ch #\E))
        (display "0: ") (display ch) (newline)
        (set! ch #\F)
        (display "1: ") (display ch) (newline)
        (set! ch #\G)
        (display "2: ") (display ch) (newline)
        (set! ch (integer->char 114))
        (display "3: ") (display ch) (newline)
        (set! ch "I'm tired of this game.")
        (display "4: ") (display ch) (newline))
    0: E
    1: F
    2: G
    3: r
    4: I'm tired of this game.
    
    > ch
    #\D
    

    Review this interaction, and explain what happens at each step.
    Here, ch is both a global and a local variable. Be sure you can explain which value is changed when and why.

Assignment Expressions For Sorting Global Data

Consider the following definitions for insert-item, insertion-sort, and sort-data:


(define insert-item
   (lambda (item ls)
      (if (or (null? ls) (<= item (car ls)))
          (cons item ls)
          (cons (car ls) (insert-item item (cdr ls)))
      )
   )
)
(define insertion-sort
   (lambda (ls)
      (if (null? ls) 
          ls
          (insert-item (car ls) (insertion-sort (cdr ls)))
      )
   )
)
(define sort-data
   (lambda ()
     (set! data (insertion-sort data))
   )
)
  1. Enter the code for insert-item, insertion-sort, and sort-data, and run the following sequence of operations:
    
    (define data '(3  1  6  -8  4))
    data
    (sort-data)
    data
    
    Explain the results you obtain.

Assignments To Parameter Lists

Next consider the following variation of insertion-sort that changes its list parameter by changing its car and cdr:


(define insertion-sort
   (lambda (ls)
      (letrec ((sort-data 
                  (lambda (lst)
                     (if (null? lst)
                         lst
                         (insert-item (car lst) (sort-data (cdr lst)))))))
          (let ((result (sort-data ls)))
             (set-car! ls (car result))
             (set-cdr! ls (cdr result))
             result
          )
      )
   )
)
  1. After defining this new insertion-sort, run the following sequence of operations:
    
    (define first '(3  1  6  -8  4))
    first
    (define second '(5 3 7 9 2 6 1 8))
    second
    (insertion-sort first)
    (insertion-sort second)
    first
    second
    
    1. Explain how this insertion-sort produces its output; that is, hypothesize how is the parameter changed.

    2. What is the purpose of the final result within the let statement; what happens if this line is omitted?
Finally, consider the following variation of the above code:

(define insertion-sort
   (lambda (ls)
      (letrec ((sort-data 
                     (lambda (lst)
                        (if (null? lst)
                            lst
                            (insert-item (car lst) (sort-data (cdr lst)))))))
          (let ((result (sort-data ls)))
             (set! ls result)
             ls
          )
      )
   )
)
  1. Run this code with the same operations as for step 4, and describe the results.

  2. Draw diagrams for the previous version of insertion-sort from step 4 to clarify further why the code with set-car! and set-cdr! works correctly -- even though the latest version with (set! ls result) does not change the global variable.




This document is available on the World Wide Web as

http://www.cs.grinnell.edu/~walker/courses/153.sp05/labs/lab-mutation.shtml

created November 12, 1997 with parts taken from a lab by John D. Stone, revised on March 8, 1998
revised January 11, 2000 by Henry M. Walker
last revised February 3, 2005
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For more information, please contact Henry M. Walker at walker@cs.grinnell.edu.