# Characters and Strings

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?`
• String predicates: `string?`
• String constructors: `make-string`, `string`, `string-append`
• String destructors: `string-ref`, `substring`
• String conversion: `list->string`, `number->string`, `string->list`
• String analysis: `string-length`,
• String comparison: `string<?`, `string<=?`, `string=?`, `string>=?`, `string>?`, `string-ci<?`, `string-ci<=?`, `string-ci=?`, `string-ci>=?`, `string-ci>?`

## Exercises

### Exercise 0: Preparation

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

### Exercise 1: Collating Sequences

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.

### 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 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 procedure `number->string`.

### 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.

c. What are the advantages of using `markup` rather than marking your code directly?

### Exercise 8: 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. `paragraph`

d. `emphasize`

### Exercise 9: Build a Page

a. 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)`.

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

## History

Tuesday, 3 October 2000 [Samuel A. Rebelsky]

• Created. Parts were probably taken from elsewhere, but I neglected to document where.

Wednesday, 7 February 2001 [Samuel A. Rebelsky]

• Deleted procedures that required repetition or recursion.
• Added new "Generate HTML" procedures.

Sunday, 18 February 2001 [Samuel A. Rebelsky]

• Changed the order of the parameters to `markup` based on experience using it.
• Updated 9b for clarification.

Monday, 16 September 2002 [Samuel A. Rebelsky]

• Added list of potentially useful procedures.
• Reformatted slightly.

Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

This document was generated by Siteweaver on Mon Dec 2 09:19:28 2002.
The source to the document was last modified on Mon Sep 16 08:52:27 2002.
This document may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2002F/Labs/strings.html`.

You may wish to validate this document's HTML ; ; Check with Bobby

Samuel A. Rebelsky, rebelsky@grinnell.edu