In this laboratory, you will further ground your understanding of what happens
behind the scenes when Scheme deals with lists and other pair-based structures.
a. Review the reading on pairs and pair structures.
b. Start DrScheme.
Draw box-and-pointer diagrams for each of the following lists:
((x) y z)
(x (y z))
((a) b (c ()))
Enter each of the following expressions into Scheme. In each case, explain why Scheme does or does not use the dot notation when displaying the value.
(cons 'a "Walker")
(cons 'a null)
(cons 'a "null")
(cons 'a "()")
(cons null 'a)
(cons null (cons null null))
Draw a box-and-pointer representation of the value of the last two expressions in the previous exercise.
What do you think that
pair? will return for each of
the following? How about
list?. Confirm you answer
experimentally and explain any that you found particularly tricky.
(cons 'a 'b)
(cons 'a (cons 'b 'c))
(cons 'a null)
(list 'a 'b 'c)
You may recall that I told you that many kinds of data are defined recursively. For example, a list is either (1) null or (2) cons of anything and a list.
Using that recursive
definition of lists, write a procedure,
(listp? val), that determines whether
or not val is a list.
You may not use
list? in your definition of
If you were able to complete the primary exercises with time to spare, you might want to consider the following problems:
listp? without using
Write a procedure,
(last pairthing) that finds the
last value in a list-like sequence of pairs. If the sequence of pairs is actually a list, return the last element of the list. If the sequence of pairs ends with a pair, return the cdr of that pair.
In solving this problem you should only step through the list once.
I usually create these pages
on the fly, which means that I rarely
proofread them and they may contain bad grammar and incorrect details.
It also means that I tend to update them regularly (see the history for
more details). Feel free to contact me with any suggestions for changes.
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; ; Check with BobbySamuel A. Rebelsky, firstname.lastname@example.org