Introduction
Factor characters are integers — Unicode code points — so numeric
<, >, = give you case-sensitive ordering directly. The
unicode vocabulary adds Unicode-aware predicates and
single-character case conversion.
Character predicates
LETTER? ( c -- ? ) ! uppercase letter
letter? ( c -- ? ) ! lowercase letter
Letter? ( c -- ? ) ! letter, either case
digit? ( c -- ? ) ! decimal digit
digit> ( c -- n ) ! the integer value of a digit char
blank? ( c -- ? ) ! whitespace
USING: unicode ;
CHAR: A LETTER? . ! => t
CHAR: a letter? . ! => t
CHAR: 7 digit? . ! => t
CHAR: 7 digit> . ! => 7
Case conversion
ch>upper and ch>lower flip a single character. The string-level
>upper and >lower apply them across every character of a string.
CHAR: a ch>upper . ! => 65 (which is CHAR: A)
"Hello" >upper . ! => "HELLO"
capitalize lowercases a whole string and then uppercases its first
character — the title case of a single word:
"hELLO" capitalize . ! => "Hello"
Character literals
CHAR: A parses to the integer code point of A. For named
characters, use the names space, tab, \n, etc.:
CHAR: A . ! => 65
CHAR: space . ! => 32
CHAR: \n . ! => 10
Symbols
When a word needs to return a tag rather than a value — say,
yes/no/maybe instead of a boolean — declare each tag as a
symbol. A symbol is a word that pushes itself when called, and
two symbols compare equal only when they are the same name.
USING: kernel ;
SYMBOLS: yes no maybe ;
yes . ! => yes
yes yes = . ! => t
yes no = . ! => f
SYMBOL: name declares one; SYMBOLS: a b c ; declares several.
Instructions
Heidi, a young Forth Alien, needs to get some work done on her
Human alphabets project and asks if you can help her program some
words she will need.
1. Compare two characters
Define compare-chars to take two characters and return the symbol
less, greater, or equal depending on whether the first is
less than, greater than, or equal to the second. Compare in a
case-sensitive manner.
CHAR: A CHAR: B compare-chars . ! => less
CHAR: B CHAR: A compare-chars . ! => greater
CHAR: A CHAR: A compare-chars . ! => equal
CHAR: A CHAR: a compare-chars . ! => less
2. Determine the size of a character
Heidi needs to know whether a character is "big" (uppercase),
"small" (lowercase), or has no size. Define size-of-char to
return big, small, or no-size.
CHAR: A size-of-char . ! => big
CHAR: a size-of-char . ! => small
CHAR: 5 size-of-char . ! => no-size
CHAR: space size-of-char . ! => no-size
3. Change the size of a character
Heidi sometimes needs to flip a character to a different size.
Define change-size-of-char to take a character and a desired
size (big or small) and return the corresponding-case
character.
CHAR: a big change-size-of-char . ! => 65 (CHAR: A)
CHAR: A small change-size-of-char . ! => 97 (CHAR: a)
4. Determine the type of a character
Heidi also needs to know what kind of character it is. Define
type-of-char to return:
-
alpha for any letter (upper or lower)
-
numeric for a decimal digit
-
space for the space character
-
newline for the newline character
-
unknown otherwise
CHAR: A type-of-char . ! => alpha
CHAR: 5 type-of-char . ! => numeric
CHAR: space type-of-char . ! => space
CHAR: \n type-of-char . ! => newline
CHAR: ! type-of-char . ! => unknown
Learning Exercise
