Introduction
Characters
Common Lisp has characters like other programming languages.
Characters are objects which represent an item in a quantity of text (e.g. a string or text stream).
Characters can be converted to and from integers. (But they are not integers - they are a distinct type.)
Characters are represented as #\ followed by its name.
The 'name' for common characters such as 'A' or 'b' or '9' or '!' are simply that: so #\A, #\b, #\9 and #\!.
Some non-graphical characters such as space or new-line have names such as #\Space and #\Newline.
The standard only requires an implementation to have 96 characters (upper and lower case Latin alphabetic characters (A-Za-z), the digits (0-9), space (#\Space), newline (#\Newline) and punctuation (e.g. !$"'(),_-./:;?+<=>#%&*@[\]{|}`^~)
However most implementations will offer more than that, including implementations which provide all Unicode characters.
Comparing characters.
Characters can be compared for equality by char= or char-equal (the latter being case-insensitive).
Characters can be ordered with functions such as char<, char>, char<=, char>= and their case-insensitive versions char-lessp and char-greaterp.
Types of characters
Characters can be categorized as different types: graphical, alphabetic, alpha-numeric, digit, upper-case or lower-case.
A character can be more than one type such as an upper-case alphabetic character.
There are predicates for each of these types: graphic-char-p, alpha-char-p, alphanumericp, digit-char-p, upper-case-p and lower-case-p.
Converting character case
Some characters can be upper or lower case and can be converted between them with char-upcase and char-downcase.
Instructions
Lewis, a young Lisp Alien, needs to get some work done on their Human alphabets project and asks if you can help them program some functions they will need.
1. Comparing characters
Lewis will need a function to compare two characters, determining if the first is greater-than, less-than or equal-to the second.
Write a function compare-chars which will take two characters and evaluate to :greater-than, :less-than, or :equal-to appropriately.
The function should compare in a case-sensitive manner.
2. Determine the "size" of the character
Lewis needs to know the if a character is "big" (a uppercase character) or "little" (a lowercase character), or "no-size" if it is neither.
Write a function size-of-char that will evaluate to :big, :small or :no-size depending upon the "size" of the character:
3. Changing the "size" of the character
Lewis may need to make a big character small, or a small character big.
Write a function change-size-of-char which takes two parameters a character and a desired size (:big or :small).
It should evaluate to the corresponding character of the correct size.
4. Determine the "type" of a character
Not only will Lewis need to know what the 'size' of the character is but also its 'type'.
Write a function type-of-char that will evaluate to :alpha if the character is an alphabetic character, :numeric if it is a numeric character, :space if it is a space character, :newline if it is a newline character and :unknown otherwise
Learning Exercise
