# Homework 12: Tallying

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

Summary: In this assignment, you will write your own higher-order procedure.

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.

## Background

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)
(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))))))
```

## Assignment

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).

## History

Thursday, 2 November 2006 [Samuel A. Rebelsky]

• Created.

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.

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Samuel A. Rebelsky, rebelsky@grinnell.edu

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