CSC 153	Grinnell College	Fall, 2007

Computer Science Fundamentals

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

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.

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.

Pick a variable that you have not used previously (e.g. myVariable). What happens when you try to assign a value to this variable:
```
>  (set! myVariable 3.1415926536)
```

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))
   )
)

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
          )
      )
   )
)

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
          )
      )
   )
)

Run this code with the same operations as for step 5, and describe the results.
Draw diagrams for the previous version of insertion-sort from step 5 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.fa07/labs/lab-mutation.shtml

created 12 November 1997 with parts taken from a lab by John D. Stone, revised on 8 March 1998 revised 11 January 2000 by Henry M. Walker last revised 30 September 2007
For more information, please contact Henry M. Walker at walker@cs.grinnell.edu.