Laboratory: Characters and Strings in Scheme
Exercise 1: Collating Sequences
As you may recall, Scheme uses a collating sequence
for the letters, assigning a sequence number to each letter.
Many implementations of Scheme, including MediaScript, use the
Unicode collating sequence. (ASCII, the American Standard Code
fo Information Interchange, is a subset of Unicode.)
a. Determine the Unicode collating-sequence numbers for the capital
letter A and for the lower-case letter a. Then determine the Unicode
collating-sequence numbers for the capital letter X and the lower-case
letter x. Do you notice any patterns?
b. Find out what Unicode character is in position 38 in the collating
sequence.
c. Do the digit characters precede or
follow the capital letters in the collating
sequence?
d. If you were designing a character set, where in the collating
sequence would you place the space character? Why?
e. What position does the space character occupy in Unicode?
f. What character occupies position 477 in Unicode?
Exercise 2: Character Predicates
a. Determine whether our implementation of Scheme considers
#\newline a whitespace character.
b. Determine whether our implementation of Scheme indicates that
capital B precedes or follows lower-case a.
c. Verify that the case-insensitive comparison operation,
char-ci<?, gives the expected result for the
previous comparison.
d. Determine whether our implementation of Scheme indicates that
#\a and #\A are the same letter. (It should
not.)
e. Find an equality predicate that returns
#t when given #\a and #\A
as parameters.
Exercise 3: String Basics
a. Write a Scheme expression to determine whether the symbol
'plaid is a string.
b. Write a Scheme expression to determine whether the character
#\A is a string.
c. Does the empty string (represented as "") count as a string?
Exercise 4: Creating Questions
Develop three ways of constructing the string "???":
one using a call to make-string,
one a call to string, and one a call to
list->string.
Exercise 5: Referencing Lengths
Here are two opposing views about the relationship between
string-length and string-ref:
- “No matter what string
str is, provided that it's not the
empty string, (string-ref str (string-length str)) will return
the last character in the string.” - “No matter what string
str is, (string-ref str
(string-length str)) is an error.”
Which, if either, of these views is correct? Why?
Exercise 6: A Simple Form Letter
Consider the following definitions:
(define cr (string #\newline))
(define letter
(string-append
"Dear " recipient ", " cr
cr
"Thank you for your submission to " magazine ". Unfortunately, we " cr
"consider the subject of your article, " article ", inappropriate for our" cr
"readership. In fact, it is probably inappropriate for any readership." cr
"Please do not contact us again, and do not bother other magazines with" cr
"this inappropriate material or we will be forced to contact the " cr
"appropriate authorities." cr
cr
"Regards," cr
"Ed I. Tor" cr))
a. What must be defined for the definition of letter to
succeed?
b. Check that the definition of letter
works by using the following associated definitions.
(define recipient "Professor Schneider")
(define magazine "College Trustee News")
(define article "Using Grinnell's Endowment to Eliminate Tuition")
c. You may note that the output is fairly ugly when you simply ask for
letter. You can get nicer output by using the
display
procedure, as in (display letter). Try doing so.
Exercise 7: Adding Quotation Marks
a. What changes are necessary to letter so that name
of the article appears in quotation marks?
b. Check your answer experimentally.
Consider the string "Department".
a. Write an expression to extract the string "Depart"
from "Department".
b. Write an expression to extract the string "part"
from "Department".
c. Write an expression to extract the string "art"
from "Department".
d. Write an expresssion to extract the string "ment"
from "Department".
e. Write an expression to extract the string "a"
from "Department".
f. Write an expression to extract the empty string
from "Department".
g. Write an expression to extract the string "Dent"
from "Department". Note that you may need to use
two calls to substring along with a call
to string-append.
h. Write an expression to extract the string "apartment"
from "Department". Once again, you may need multiple
calls.
Exercise 9: Deleting Letters
Write a procedure, (delete-char
str pos),
that creates a new string by deleting the character at position
pos from str.
For example,
> (delete-char "starlings" 8)
"starling"
> (delete-char "starling" 4)
"staring"
> (delete-char "staring" 2)
"string"
> (delete-char "string" 2)
"sting"
> (delete-char "sting" 1)
"sing"
> (delete-char "sing" 3)
"sin"
> (delete-char "sin" 0)
"in"
> (delete-char "in" 1)
"i"
> (delete-char "i" 0)
""
Example suggested by Jonathan Rebelsky.
Exercise 10: Strings as collections of
characters
Write procedures to do the following:
(a)
(string-sum str)
converts each character in str to an integer and
computes the sum of those integers. For example,
> (char->integer #\h)
104
> (char->integer #\i)
105
> (string-sum "hi")
209
(b)
(string-palindrome? str)
produces #t if str reads the same
forwards and backwards; #f otherwise. For example,
> (string-palindrome? "ana")
#t
> (string-palindrome? "anna")
#t
> (string-palindrome? "hannah")
#t
> (string-palindrome? "banana")
#f
Note: Many strategies are possible. Either list
recursion (after converting the string to a list of characters)
or numeric recursion (with string-ref)
may help you solve these problems.
