Homework #8

This problem requires Lecture 6 and book section 1.2.

Exercise 1.15, problems 1-5, p.24-25 - (60 minutes)
Here's a whole plate full of Scheme functions for you to define. You'll need lots of practice writing Scheme functions to get through this class. Here's a start. See Exercises 1.16 and 1.17 if you want more practice.

1. (duple n x) returns a list containing n copies of x.

	> (duple 2 3)
	(3 3)
	> (duple 4 '(ho ho))
	((ho ho) (ho ho) (ho ho) (ho ho))
	> (duple 0 '(blah))
	()

2. (invert lst), where lst is a list of 2-lists (lists of length two), returns a list with each 2-list reversed.

	>(invert '((a 1) (a 2) (b 1) (b 2)))
	((1 a) (2 a) (1 b) (2 b))

	> (filter-in number? '(a 2 (1 3) b 7))
	(2 7)
	> (filter-in symbol? '(a (b c) 17 foo))
	(a foo)

4. (every? pred lst) returns #f if any element of lst fails to satisfy pred, and returns #t otherwise.

	> (every? number? ' (a b c d 3 e))
	#f
	> (every? number? '(1 2 3 5 4))
	#t

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5. (exists? pred lst) returns #t if any element of lst satisfies pred, and returns #f otherwise.

	> (exists? number? '(a b c 3 e))
	#t
	> (exists? number? '(a b c d e))
	#f

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YOU MUST NAME YOUR FUNCTIONS WITH THE SAME NAMES THEY HAVE IN THE BOOK (i.e., duple, invert, filter-in, every?, and exists?).

Debugging Tip: DrScheme has a trace facility that will display when a function is called, what its parameters were, and what its return value was. The syntax for turning on tracing for a particular function is (trace <function>). So, for example, you could turn on debugging for your duple function by typing (trace duple). You have to explicitly turn on tracing for each function you want to trace. The (untrace <function>) operator will likewise turn tracing off for a particular function.

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Last updated at 7:07 am on Friday, September 17, 2004.