3.4 Strings (Unicode)

A string is a fixed-length array of characters. It prints using doublequotes, where doublequote and backslash characters within the string are escaped with backslashes. Other common string escapes are supported, including \n for a linefeed, \r for a carriage return, octal escapes using \ followed by up to three octal digits, and hexadimal escapes with \u (up to four digits). Unprintable characters in a string normally shown with \u when the string is printed.

Reading Strings in PLT Scheme Reference documents the fine points of the syntax of strings.

The display procedure directly writes the characters of a string to the current output port (see Input and Output), in contrast to the string-constant syntax used to print a string result.

Examples:

> "Apple"

"Apple"

> "\u03BB"

"λ"

> (display "Apple")

Apple

> (display "a \"quoted\" thing")

a "quoted" thing

> (display "two\nlines")

two   lines

> (display "\u03BB")

λ

A string can be mutable or immutable; strings written directly as expressions are immutable, but most other strings are mutable. The make-string procedure creates a mutable string given a length and optional fill character. The string-ref procedure accesses a character from a string (with 0-based indexing); the string-set! procedure changes a character in a mutable string.

Examples:

> (string-ref "Apple" 0)

#\A

> (define s (make-string 5 #\.))

> s

"....."

> (string-set! s 2 #\λ)

> s

"..λ.."

String ordering and case operations are generally locale-independent; that is, they work the same for all users. A few locale-dependent operations are provided that allow the way that strings are case-folded and sorted to depend on the end-user’s locale. If you’re sorting strings, for example, use string<? or string-ci<? if the sort result should be consistent across machines and users, but use string-locale<? or string-locale-ci<? if the sort is purely to order strings for an end user.

Examples:

> (string<? "apple" "Banana")

#f

> (string-ci<? "apple" "Banana")

#t

> (string-upcase "Straße")

"STRASSE"

> (parameterize ([current-locale "C"])       (string-locale-upcase "Straße"))

"STRAßE"

For working with plain ASCII, working with raw bytes, or encoding/decoding Unicode strings as bytes, use byte strings.

Strings in PLT Scheme Reference provides more on strings and string procedures.