2.9 KiB
Characters
A Racket character corresponds to a Unicode scalar value. Roughly, a scalar value is an unsigned integer whose representation fits into 21 bits, and that maps to some notion of a natural-language character or piece of a character. Technically, a scalar value is a simpler notion than the concept called a “character” in the Unicode standard, but it’s an approximation that works well for many purposes. For example, any accented Roman letter can be represented as a scalar value, as can any common Chinese character.
Although each Racket character corresponds to an integer, the character
datatype is separate from numbers. The char->integer and
integer->char procedures convert between scalar-value numbers and the
corresponding character.
A printable character normally prints as #\ followed by the
represented character. An unprintable character normally prints as #\u
followed by the scalar value as hexadecimal number. A few characters are
printed specially; for example, the space and linefeed characters print
as #\space and #\newline, respectively.
+[missing] in [missing] documents the fine points of the syntax of characters.
Examples:
> (integer->char 65)
#\A
> (char->integer #\A)
65
> #\λ
#\λ
> #\u03BB
#\λ
> (integer->char 17)
#\u0011
> (char->integer #\space)
32
The display procedure directly writes a character to the current
output port see \[missing\], in contrast to the character-constant
syntax used to print a character result.
Examples:
> #\A
#\A
> (display #\A)
A
Racket provides several classification and conversion procedures on characters. Beware, however, that conversions on some Unicode characters work as a human would expect only when they are in a string (e.g., upcasing “ß” or downcasing “Σ”).
Examples:
> (char-alphabetic? #\A)
#t
> (char-numeric? #\0)
#t
> (char-whitespace? #\newline)
#t
> (char-downcase #\A)
#\a
> (char-upcase #\ß)
#\ß
The char=? procedure compares two or more characters, and char-ci=?
compares characters ignoring case. The eqv? and equal? procedures
behave the same as char=? on characters; use char=? when you want to
more specifically declare that the values being compared are characters.
Examples:
> (char=? #\a #\A)
#f
> (char-ci=? #\a #\A)
#t
> (eqv? #\a #\A)
#f
+[missing] in [missing] provides more on characters and character procedures.