Characters and Strings in Scheme

Contents:

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Collating Sequences
  • Exercise 2: A Control Predicate
  • Exercise 3: String Basics
  • Exercise 4: Creating Questions
  • Exercise 5: Referencing Lengths
  • Exercise 6: Generating Headings
  • Exercise 7: Marking Text
  • Exercise 8: Markup, Revisited
• For Those with Extra Time
  • Other Markup
  • Build a Page
  • A Parameterized Page

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?
• 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>?

Exercises

Exercise 0: Preparation

a. If you have not done so already, you may want to skim the reading on characters and the reading on strings.

b. Start DrScheme.

Exercise 1: Collating Sequences

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?

Exercise 2: A Control Predicate

In ASCII, the collating-sequence numbers of the control characters are 0 through 31 and 127. Define a predicate char-control? that returns #t if its argument is a control character, #f otherwise.

Note that char-control? takes a character, not an integer, as a parameter.

Exercise 3: String Basics

a. Is the symbol 'hyperbola a string?

b. Is the character #\A a string?

c. Does the empty string count as a string?

Exercise 4: Creating Questions

Suggest 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: Generating Headings

Write a procedure, (heading level text) that generates a string that contains a string for an HTML heading of the appropriate level. For example,

> (heading 2 "Exercise 6")
"<h2>Exercise 6</h2>"
> (heading 4 "History")
"<h4>History</h4>"

You may find it useful to use the procedures number->string and string-append.

Note that you can check special characters in your output by using (display exp), which shows exp without the quotation marks and with the special control characters (such as newline) as the character the represent, rather than a special control sequence.

Exercise 7: Marking Text

a. Write a procedure, (markup tag text) that surrounds text with the given tag. For example.

> (markup "p" "Hi There")
"<p>Hi There</p>"
> (markup "strong" "Wicked Neat!")
"<strong>Wicked Neat!</strong>"

b. Use markup, string-append, and any other procedures you deem appropriate to generate the following HTML:

<p>
Sam says <q>Scheme is <strong>Wicked Neat!</strong></q>
</p>

Note that you may want to use the character #\newline for new lines.

d. Write a (paragraph text) procedure using markup

Exercise 8: Markup, Revisited

a. Write a variant of markup, (mark tag attribute-list text), that surrounds text with the appropriate tags and includes an appropriate set of attributes in the opening tag. The attribute parameter should be a list of lists. The car of each component list is a string that gives an attribute name. The cdr of each component list is a string that gives a value. For example,

> (mark "a" '(("href" "http://www.cs.grinnell.edu")) "CS")
"<a href=\"http://www.cs.grinnell.edu\">CS</a>"
> (mark "p" null "Hello.")
"<p>Hello</p>"

b. Extend mark so that it can take an arbitrary number of parameters. It should concatenate together all the parameters after attribute-list before surrounding them with the appropriate tag.

c. Why might you use Scheme rather than a text editor to build a Web page?

For Those with Extra Time

Other Markup

Use markup to implement the following procedures, each of which takes one argument (some text) and generates HTML for appropriately formatted text.

a. bold

b. strong

c. emphasize

Build a Page

Using the previous procedures, write a procedure, page, of no arguments that builds a simple HTML page of your choice. Your procedure will begin

(define page
  (lambda ()
    instructions-for-building-the-page)))

You can call the procedure with (page).

A Parameterized Page

Write a procedure, (rant-page subject), that builds an HTML page that rants non-specifically about subject. For example, if subject were days in which the temperature is less than zero degrees Fahrenheit the body of the page might read

I hate days in which the temperature is less than zero degrees Fahrenheit. You probably hate days in which the temperature is less than zero degrees Fahrenheit, too.

Similarly, if subject were malicious computers, the page might read

I hate malicious computers. You probably hate malicious computers, too.

You may instead make the page say nice things, if you would prefer.