► |  | Guide: PLT Scheme

1 | Welcome to PLT Scheme
2 | Scheme Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units (Components)
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Performance
18 | Running and Creating Executables
19 | Compilation and Configuration
20 | More Libraries
21 | Dialects of Scheme
| Bibliography
| Index

▼ | 3 | Built-In Datatypes

3.1 | Booleans
3.2 | Numbers
3.3 | Characters
3.4 | Strings (Unicode)
3.5 | Bytes and Byte Strings
3.6 | Symbols
3.7 | Keywords
3.8 | Pairs and Lists
3.9 | Vectors
3.10 | Hash Tables
3.11 | Boxes
3.12 | Void and Undefined

Version: 4.1.4
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#### 3.3`` Characters

A Scheme 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 Chinese character.

Although each Scheme 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 Reference: PLT Scheme 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 Input and Output), in contrast to the character-constant syntax used to print a character result.

Examples:
> #\A
#\A
> (display #\A)
A

Scheme 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

Characters in Reference: PLT Scheme provides more on characters and character procedures.

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