Draw box-and-pointer diagrams for each of the following lists:
((x) y z)(x (y z))((a) b (c ()))Have DrScheme evaluate each of the following expressions. 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 null 'a)(cons null (cons null null))Draw a box-and-pointer representation of the value of each expression in the previous exercise.
Define and test a procedure named cons-cell-count that takes
any Scheme value and determines how many boxes would appear in its
box-and-pointer diagram. (The data structure that is represented by such
a
box, or the region of a computer's memory in which such a structure is
stored is called a cons cell. Every time the cons
procedure is used, explicitly or implicitly, in the construction of a
Scheme value, a new cons cell is allocated, to store information about the
car and the cdr. Thus cons-cell-count also tallies the
number
of times cons was invoked during the construction of its
argument.)
For example, the structure in the last box-and-pointer diagram in the reading
contains seven cons-cells, so when you apply cons-cell-count
to that structure, it should return 7. On the other hand, the string
"sample" contains no cons-cells, so the value of
(cons-cell-count "sample") is 0.
Use cons-cell-count to find out how many cons cells are
needed
to construct the list (0 (1 (2 (3 (4))))). Draw a
box-and-pointer diagram of this list to check the answer.
This document is available on the World Wide Web as
http://www.cs.grinnell.edu/~gum/courses/151/labs/pairs.xhtml
created February 21, 1997
last revised August 12, 2001
John David Stone (stone@cs.grinnell.edu) and Ben Gum (gum@cs.grinnell.edu)