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 hexadecimal escapes with \u (up to four digits). Unprintable characters in a string are normally shown with \u when the string is printed.

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