Summary: In this laboratory, you will enhance your understanding of strings in Scheme and learn some new procedures as you focus on a useful task: turning regular phrases into those a pirate might say.
Supplies:
-
slams.ss, Sam's Library for Amazingly Munging Strings.
-
pirate.ss, a sample application.
Useful Procedures:
join,
map,
random,
split,
string=?,
Contents:
a. Start DrScheme.
b. At the top of the definitions pane, enter
(load "/home/rebelsky/Web/Scheme/slams.ss")
Document and write a procedure, (pirate-greeting word)
that returns an appropriate pirate greeting (e.g., Ahoy
) if
word is "Hello" and returns word
if word is anything else.
Extend pirate-greeting so that it returns the greeting
for more than just "Hello". It shouldn't return the greeting
you have selected all the time, but it should probably return it for
"Hi" and "Greetings".
Write a procedure, (pirate-guy word) that returns a "piratey" word for a man if word is "man".
Most pirates are not so boring as to use the same word for man
at every step. They might use bilge rat
, landlubber
, bastard
, and many other things, depending on their mood and the situation. Hence, we should have pirate-guy make different choices depending on the computer's mood
.
Fortunately, Scheme provides a random procedure that
can help. The expression (random number) returns
an integer between 0 and number-1. You can use this procedure
to select between two strings with
(if (= (random 2) 0) "bilge rat" "landlubber")
a. Execute the preceding expression a few times to see what happens.
b. What happens if you change the 2 to a 4 in the example?
c. Rewrite pirate-guy so that it alternates between three or more terms.
Combine the ideas from the previous exercises to write a single procedure,
(pirate-word word) that converts word to
a word in pirate-speak, at least for some words. For example, it should
convert "Hello" to "Ahoy" and "man"
to some appropriate appellation.
Right now, pirate-word changes only one word. What if we'd
like to change multiple words? We can use one of the really-cool Scheme
procedures, map. The map procedure takes two
arguments, a procedure and a list, and it applies the procedure to every
element of the list.
That description may not make much sense to you until you've tried it.
a. See what happens when you write
(map pirate-word (list "one" "man" "and" "another" "man" "played" "dice"))
b. Play with enough examples that you're confident that you understand what map does.
Scheme's map procedure makes life much easier. However, it is still fairly painful to have to write phrases as a list of words. Unfortunately, Scheme does not come with a procedure for extracting all the words in a phrase. Fortunately, I've spent the time writing such a procedure. I call it split. (No, it has nothing to do with bowling.) Its inverse is join.
a. Try applying split to a variety of strings and see if you can figure out what it does. Here are a few you might try.
-
"hello"
-
"Hello. What is your name?"
b. Try applying join to some lists of words. What does it seem to do?
c. How could you combine split, join, map, and pirate-word to piratize the phrase "Hello. I just saw a man laugh at another man."
If you figured out the previous problem, you probably came up with an answer
like
(join (map pirate-word (split "...")))
a. It is painful to have to type all of that each time we want to piratize a phrase. Turn that expression into a procedure, (piratize phrase).
b. Try it on a number of examples.
Some pirates like to add an extra phrase to the end of a sentence, such as
Saavy?
. Figure out how to extend piratize to add some piratey phrases to the ends of some sentences.
Extend your procedures as far as you are willing. You will turn in these procedures as homework 2.
Wednesday, 17 September 2003 [Samuel A. Rebelsky]
Friday, 19 September 2003 [Samuel A. Rebelsky]
;
Check with Bobby