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Summary: In these exercises, you will explore a number of the standard Scheme procedures for handling characters and strings. You will also explore an application of these procedures for marking up text.
Contents:
Useful Procedures and Notation:
- Constant notation:
#\ch (character constants)
"string" (string constants).
- Character constants:
#\a (lowercase a) ... #\z (lowercase z);
#\A (uppercase A) ... #\Z (uppercase Z);
#\0 (zero) ... #\9 (nine);
#\space (space);
#\newline (newline);
and
#\? (question mark).
- Character conversion:
char->integer,
integer->char,
char-downcase, and
char-upcase
- Character predicates:
char?,
char-alphabetic?,
char-numeric?,
char-lower-case?,
char-upper-case?,
char-whitespace?,
char<?,
char<=;?,
char=?,
char>=;?,
char>?,
char-ci<?,
char-ci<=;?,
char-ci=?,
char-ci>=;?, and
char-ci>?.
- String predicates:
string?
- String constructors:
make-string,
string,
string-append
- String extractors:
string-ref,
substring
- String conversion:
list->string,
number->string,
string->list
- String analysis:
string-length,
- String comparison (links may not work):
string<?,
string<=?,
string=?,
string>=?,
string>?,
string-ci<?,
string-ci<=?,
string-ci=?,
string-ci>=?,
string-ci>?
a. If you have not done so already, you may also want to open separate
tabs with
the reading on characters and
the reading on strings.
b. If you have not done so already, you may want to skim
Section 6.3.5 of the Scheme Report.
c. Start DrScheme.
As you may recall, Scheme uses a collating sequence for the
letters, assigning a sequence number to each letter. DrScheme uses
the ASCII collating sequence.
a. Determine the ASCII collating-sequence numbers for the capital letter A and
for the lower-case letter a.
b. Find out what ASCII character is in position 38 in the collating sequence.
c. Do the digit characters precede or follow the capital
letters in the ASCII 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 ASCII?
a. Determine whether our implementation of Scheme considers #\newline a whitespace character.
b. Determine whether our implementation of Scheme indicates that capital
B precedes lower-case a.
c. Determine whether our implementation of Scheme indicates that lower-case
a precedes capital B.
d. Verify that the case-insensitive comparison operation (char-ci<?) gives the expected result for the previous two tests.
e. Determine whether our implementation of Scheme indicates that #\a and
#\A are the same letter. (It should not.)
f. Find an equality predicate that returns #t when given
#\a and #\A as parameters.
a. Write a Scheme expression to determine whether the symbol 'hyperbola 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?
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.
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?
Consider the definition
(define like
(string-append
"I like "
person
" because "
person
" is "
adjective
"."))
a. What other values must be defined in order for this definition to work?
b. What type must those values have?
c. Suppose you had previously defined person as
"Daniel" and adjective as
"cheerful". What do you expect the value of
like to be?
d. Confirm your previous answer experimentally.
One criticism of the like definition in the previous exercise
is that it takes a lot of lines. We could define a similar sentence as
follows:
(define tunes
(string-append "I listen to " band " because their music is " adjective "."))
a. What are the comparative advantages and disadvantages of the single-line
sentence-building definition?
b. Define band and adjective in such a way that
tunes can be successfully defined.
We can, of course, use a similar technique to build longer form letters.
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. Confirm that the definition of letter
works by using the following sub-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.
a. What changes are necessary to letter so that name
of the article appears in quotation marks?
b. Confirm your answer experimentally.
a. Create a file, sam.ss, with the following lines:
(define recipient "Mr. Rebelsky")
(define magazine "Liberal Arts Letters")
(define article "Why Every Faculty Member Should Take Introductory Computer Science")
b. Create a separate file, letter.ss that contains the definition
of cr and the updated definition of letter from
the previous exercises.
c. In the definitions window, type the following
(load "sam.ss")
(load "letter.ss")
(display letter)
d. What do you expect to happen when you click Run?
e. Confirm your answer experimentally.
f. What ideas does this exercise suggest to you?
Create anotehr form of form letter.
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