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CHAPTER 18: RECURSION ON TREES
- Corrections
- page 130, lines 1 - 7: Change indentation from
(defun bstreep (tree)
"A bstree is either an element, or a three-member list,
the first of which is an element."
(or (typep tree 'util:element)
(and (listp tree)
(= (length tree) 3)
(typep (first tree) 'util:element))))
to
(defun bstreep (tree)
"A bstree is either an element, or a three-member list,
the first of which is an element."
(or (typep tree 'util:element)
(and (listp tree)
(= (length tree) 3)
(typep (first tree) 'util:element))))
- Page 298, lines 15 - 18: Change
(defun lhs (rule)
"Returns the right-hand side of the RULE."
(check-type rule rule)
(first rule))
to
(defun lhs (rule)
"Returns the left-hand side of the RULE."
(check-type rule rule)
(first rule))
- Notes
-
- Read Chapter 18.
- Do Exercise 18.1 if you are not already comfortable with the
difference between
eql and equal.
- Do Exercises 18.2 through 18.4 if you need to in order to
feel more comfortable with recursion on trees. In any case, note the
typecase function.
- Do Exercise 18.5. Try both
lisp:equal and
lisp:tree-equal on trees at least one of whose leaves is
a string.
- For Exercise 18.6, explore the differences between
equal and equalp. I personally often use
equal, but seldom use equalp.
- For Exercise 18.7, look in Section B.13.4. The functions
that are not explicitly called destructive are not.
- Do Exercises 18.8 - 18.11 for your own understanding.
- Don't bother doing Exercises 18.12 - 18.22.
- Do Exercises 18.23 and 18.24. Submit a file named
ch18.cl with flatten and
flatten2 defined in the ch18 package.
- Do Exercises 18.25 - 18.27. Submit your updated match.cl file.
Please remember to revise the line "This file satisfies the exercises
through Exercise ???"
Exercise 18.25 is probably not worded as well as it might be.
You should rewrite match and substitute so
that in calls like (match pat tree) and (substitute
pat subs), pat and
tree can be arbitrary trees, though only
pat can contain variables. That is, previously
match worked on two lists, one of which could contain
variables; now it should work on two trees, one of which can contain
variables. Remember that, whereas the base case of lists is
nil, the base case of trees is any atom.
Here are some examples of attempts to match trees:
> (match 'a 'a)
((T T))
> (match 'a 'b)
NIL
> (match '?x 'a)
((?X A) (T T))
> (match '(?x) 'a)
NIL
> (match '?x '(a))
((?X (A)) (T T))
> (match '(a ?x c) '(a b c))
((?X B) (T T))
> (match '(a (?x) c) '(a b c))
NIL
> (match '(a ?x c) '(a (b) c))
((?X (B)) (T T))
> (match '(a (?x) c) '(a (b) c))
((?X B) (T T))
- When you have submitted correct exercises through this
chapter, you will have earned a grade of C+ (above a 2.0).
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