3.3 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.
Reading Characters in The Racket Reference documents the fine points of the syntax of characters.
> (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 Input and Output), in contrast to the character-constant syntax used to print a character result.
> #\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”).
> (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.