Fundamentals of Computer Science I (CS151.01 2006F)

Homework 12: Tallying

This homework assignment is also available in PDF.

Assigned: Friday, 3 November 2006
Due: Tuesday, 7 November 2006
No extensions!

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 12.

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 map and to factor out the process of checking all the values in a list with list-of?.

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)
      ((null? lst) 0)
      ((symbol? (car lst)) (+ 1 (tally-symbols (cdr lst))))
      (else (tally-symbols (cdr lst))))))
(define tally-odds
  (lambda (lst)
      ((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 pred? holds.

b. Rewrite tally-symbols and tally-odds using 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).



Thursday, 2 November 2006 [Samuel A. Rebelsky]


Disclaimer: 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.

This document was generated by Siteweaver on Thu Nov 30 21:42:53 2006.
The source to the document was last modified on Thu Nov 2 21:25:04 2006.
This document may be found at

You may wish to validate this document's HTML ; Valid CSS! ; Creative Commons License

Samuel A. Rebelsky,

Copyright © 2006 Samuel A. Rebelsky. This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.