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Summary: In this assignment, you will write your own higher-order procedure.
Purposes: To help you think about higher-order procedures and how they might help with control.
Expected Time: One to two hours.
Collaboration: You may work in a group of any size between one and four, inclusive. You may consult others outside your group, provided you cite those others. You need only submit one assignment per group.
Submitting: Email me your work, using a subject of CSC151 Homework 14.
Warning: So that this exercise is a learning assignment for everyone, I may spend class time publicly critiquing your work.
As you've seen in our discussions this past week, one of the
key ideas in functional programming is that you can factor out
common control structures. We've seen it possible to factor out the
process of building a new list by recursing over the list with
and to factor out the process of checking all the values in a list with
Here's another common task: Counting values that match some predicate. We've written procedures that count the number of symbols in a list and that count the number of odd numbers in a list of numbers.
(define tally-symbols (lambda (lst) (cond ((null? lst) 0) ((symbol? (car lst)) (+ 1 (tally-symbols (cdr lst)))) (else (tally-symbols (cdr lst)))))) (define tally-odds (lambda (lst) (cond ((null? lst) 0) ((odd? (car lst)) (+ 1 (tally-odds (cdr lst)))) (else (tally-odds (cdr lst))))))
a. Write a procedure,
(tally pred? lst), that counts the
number of values in
lst for which
tally. That is, your definitions should look
something like the following:
(define tally-symbols (lambda (lst) (tally ...)))
c. Write a procedure,
tally-as, which takes a list of
integers (representing grades) as a parameter and returns the number of
values that are 90 and above. You should not verify the preconditions
of the procedure (that is, do not check that it's a list of integers).
d. Write a procedure
better-tally-odds, which takes a list
of arbitrary Scheme values as a parameter and returns a count of the
number of times an odd integer appears in the list.
> (tally-odds (list 'a 'b 3)) odd?: expects argument of type *lt;integer>; given a > (better-tally-odds (list 'a 'b 3 "hello" list -1)) 2
You should write
better-tally-odds so that its body
is a call to
tally with an appropriate predicate as the
e. [Optional] Write
tally-odds without using a
lambda. Hint: You might want to use
Thursday, 2 November 2006 [Samuel A. Rebelsky]
Tuesday, 17 April 2007 [Samuel A. Rebelsky]
tally-oddsand the related procedures.
better-tally-odds) and e (definitions without lambdas).
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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