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Characters are objects that represent printed characters, such as letters and digits.(5)
Characters are written using the notation #\character or
#\character-name. For example:
#\a ; lowercase letter #\A ; uppercase letter #\( ; left parenthesis #\space ; the space character #\newline ; the newline character
Case is significant in #\character, but not in
#\character-name. If character in
#\character is a letter, character must be followed
by a delimiter character such as a space or parenthesis. Characters
written in the #\ notation are self-evaluating; you don't need to
quote them.
A character name may include one or more bucky bit prefixes to indicate that the character includes one or more of the keyboard shift keys Control, Meta, Super, Hyper, or Top (note that the Control bucky bit prefix is not the same as the ASCII control key). The bucky bit prefixes and their meanings are as follows (case is not significant):
Key Bucky bit prefix Bucky bit --- ---------------- --------- Meta M- or Meta- 1 Control C- or Control- 2 Super S- or Super- 4 Hyper H- or Hyper- 8 Top T- or Top- 16
For example,
#\c-a ; Control-a #\meta-b ; Meta-b #\c-s-m-h-a ; Control-Meta-Super-Hyper-A
The following character-names are supported, shown here with their ASCII equivalents:
Character Name ASCII Name -------------- ---------- altmode ESC backnext US backspace BS call SUB linefeed LF page FF return CR rubout DEL space tab HT
In addition, #\newline is either #\linefeed or
#\return, depending on the operating system that Scheme is
running under. All of the standard ASCII names for non-printing
characters are supported:
NUL SOH STX ETX EOT ENQ ACK BEL BS HT LF VT FF CR SO SI DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC FS GS RS US DEL
(char->name #\a) => "a" (char->name #\space) => "Space" (char->name #\c-a) => "C-a" (char->name #\control-a) => "C-a"
Slashify?, if specified and true, says to insert the necessary
backslash characters in the result so that read will parse it
correctly. In other words, the following generates the external
representation of char:
(string-append "#\\" (char->name char #t))
If slashify? is not specified, it defaults to #f.
name->char signals
an error.
(name->char "a") => #\a (name->char "space") => #\Space (name->char "c-a") => #\C-a (name->char "control-a") => #\C-a
#t if the specified characters are have the appropriate
order relationship to one another; otherwise returns #f. The
-ci procedures don't distinguish uppercase and lowercase letters.
Character ordering follows these rules:
(char<? #\0 #\9) returns
#t.
(char<? #\A
#\B) returns #t.
(char<? #\a
#\b) returns #t.
In addition, MIT Scheme orders those characters that satisfy
char-standard? the same way that ASCII does. Specifically,
all the digits precede all the uppercase letters, and all the upper-case
letters precede all the lowercase letters.
Characters are ordered by first comparing their bucky bits part and then their code part. In particular, characters without bucky bits come before characters with bucky bits.
#t if object is a character; otherwise returns
#f.
(char-ci=? char
char2).
char->digit returns #f.
Note that this procedure is insensitive to the alphabetic case of char.
(char->digit #\8) => 8 (char->digit #\e 16) => 14 (char->digit #\e) => #f
digit->char
returns #f.
(digit->char 8) => #\8 (digit->char 14 16) => #\E
An MIT Scheme character consists of a code part and a bucky bits part. The MIT Scheme set of characters can represent more characters than ASCII can; it includes characters with Super, Hyper, and Top bucky bits, as well as Control and Meta. Every ASCII character corresponds to some MIT Scheme character, but not vice versa.(6)
MIT Scheme uses a 7-bit ASCII character code with 5 bucky bits. The least significant bucky bit, Meta, is stored adjacent to the MSB of the character code, allowing the least significant 8 bits of a character object to be interpreted as ordinary ASCII with a meta bit. This is compatible with standard practice for 8-bit characters when meta bits are employed.
char-code and char-bits to
extract the code and bucky bits from the character. If 0 is
specified for bucky-bits, make-char produces an ordinary
character; otherwise, the appropriate bits are turned on as follows:
1 Meta 2 Control 4 Super 8 Hyper 16 Top
For example,
(make-char 97 0) => #\a (make-char 97 1) => #\M-a (make-char 97 2) => #\C-a (make-char 97 3) => #\C-M-a
(char-bits #\a) => 0 (char-bits #\m-a) => 1 (char-bits #\c-a) => 2 (char-bits #\c-m-a) => 3
(char-code #\a) => 97 (char-code #\c-a) => 97
char->integer returns the character code representation for
char. integer->char returns the character whose character
code representation is k.
In MIT Scheme, if (char-ascii? char) is true, then
(eqv? (char->ascii char) (char->integer char))
However, this behavior is not required by the Scheme standard, and code that depends on it is not portable to other implementations.
These procedures implement order isomorphisms between the set of
characters under the char<=? ordering and some subset of the
integers under the <= ordering. That is, if
(char<=? a b) => #t and (<= x y) => #t
and x and y are in the range of char->integer,
then
(<= (char->integer a)
(char->integer b)) => #t
(char<=? (integer->char x)
(integer->char y)) => #t
Note: if char is a character constant for which
char->integer returns an integer strictly less than 256, then the
compiler will constant-fold the call, replacing it with the
corresponding integer. Likewise, if k is an integer constant
strictly less than 256, the compiler will constant-fold a call to
integer->char, replacing it with the corresponding character.
This is a very useful way to denote unusual character constants or
ASCII codes.
char->integer is defined to be the exact
non-negative integers that are less than the value of this variable
(exclusive).
MIT Scheme internally uses ASCII codes for I/O, and stores character objects in a fashion that makes it convenient to convert between ASCII codes and characters. Also, character strings are implemented as byte vectors whose elements are ASCII codes; these codes are converted to character objects when accessed. For these reasons it is sometimes desirable to be able to convert between ASCII codes and characters.
Not all characters can be represented as ASCII codes. A character that has an equivalent ASCII representation is called an ASCII character.
#f.
In the current implementation, the characters that satisfy this
predicate are those in which the Control, Super, Hyper, and Top bucky
bits are turned off. All characters for which the char-bits
procedure returns 0 or 1 (i.e. no bucky bits, or just
Meta) count as legal ASCII characters.
condition-type:bad-range-argument is signalled if char
doesn't have an ASCII representation.
MIT Scheme's character-set abstraction is used to represent groups of characters, such as the letters or digits. Character sets may contain only ASCII characters; in the future this may be changed to allow the full range of characters.
There is no meaningful external representation for character sets; use
char-set-members to examine their contents. There is (at
present) no specific equivalence predicate for character sets; use
equal? for this purpose.
#t if object is a character set; otherwise returns
#f.(7)
char-set-members.
Alphabetic characters are the 52 upper and lower case letters.
Numeric characters are the 10 decimal digits. Alphanumeric
characters are those in the union of these two sets. Whitespace
characters are #\space, #\tab, #\page,
#\linefeed, and #\return. Graphic characters are
the printing characters and #\space. Standard characters
are the printing characters, #\space, and #\newline.
These are the printing characters:
! " # $ % & ' ( ) * + , - . /
0 1 2 3 4 5 6 7 8 9
: ; < = > ? @
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
[ \ ] ^ _ `
a b c d e f g h i j k l m n o p q r s t u v w x y z
{ | } ~
#t if the char is in char-set; otherwise
returns #f.
char-set returns an empty
character set.
(apply char-set
chars).
predicate->char-set creates and returns a character set
consisting of the ASCII characters for which predicate is
true.
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