3.3 Strings

Strings (Unicode) in Guide: PLT Scheme introduces strings.

A string is a fixed-length array of characters.

A string can be mutable or immutable. When an immutable string is provided to a procedure like string-set!, the exn:fail:contract exception is raised. String constants generated by the default reader (see Reading Strings) are immutable.

Two strings are equal? when they have the same length and contain the same sequence of characters.

A string can be used as a single-valued sequence (see Sequences). The characters of the string serve as elements of the sequence. See also in-string.

See also: immutable, symbol->string, bytes->string/utf-8.

3.3.1 String Constructors, Selectors, and Mutators

(string? v) → boolean?

v : any/c

Returns #t if v is a string, #f otherwise.

Examples:

> (string? "Apple")

#t

> (string? 'apple)

#f

(make-string k [char]) → string?

k : exact-nonnegative-integer?

char : char? = #\nul

Returns a new mutable string of length k where each position in the string is initialized with the character char.

Examples:

> (make-string 5 #\z)

"zzzzz"

(string char ...) → string?

char : char?

Returns a new mutable string whose length is the number of provided chars, and whose positions are initialized with the given chars.

Examples:

> (string #\A #\p #\p #\l #\e)

"Apple"

(string->immutable-string str) → (and/c string? immutable?)

str : string?

Returns an immutable string with the same content as str, returning str itself if str is immutable.

(string-length str) → exact-nonnegative-integer?

str : string?

Returns the length of str.

Examples:

> (string-length "Apple")

5

(string-ref str k) → char?

str : string?

k : exact-nonnegative-integer?

Returns the character at position k in str. The first position in the string corresponds to 0, so the position k must be less than the length of the string, otherwise the exn:fail:contract exception is raised.

Examples:

> (string-ref "Apple" 0)

#\A

(string-set! str k char) → void?

str : (and/c string? (not/c immutable?))

k : exact-nonnegative-integer?

char : char?

Changes the character position k in str to char. The first position in the string corresponds to 0, so the position k must be less than the length of the string, otherwise the exn:fail:contract exception is raised.

Examples:

> (define s (string #\A #\p #\p #\l #\e))

> (string-set! s 4 #\y)

> s

"Apply"

(substring str start [end]) → string?

str : string?

start : exact-nonnegative-integer?

end : exact-nonnegative-integer? = (string-length str)

Returns a new mutable string that is (- end start) characters long, and that contains the same characters as str from start inclusive to end exclusive. The start and end arguments must be less than the length of str, and end must be greater than or equal to str, otherwise the exn:fail:contract exception is raised.

Examples:

> (substring "Apple" 1 3)

"pp"

> (substring "Apple" 1)

"pple"

(string-copy str) → string?

str : string?

Returns (substring str 0).

(string-copy!   dest             dest-start             src            [ src-start             src-end])   →   void?

dest : (and/c string? (not/c immutable?))

dest-start : exact-nonnegative-integer?

src : string?

src-start : exact-nonnegative-integer? = 0

src-end : exact-nonnegative-integer? = (string-length src)

Changes the characters of dest starting at position dest-start to match the characters in src from src-start (inclusive) to src-end (exclusive). The strings dest and src can be the same string, and in that case the destination region can overlap with the source region; the destination characters after the copy match the source characters from before the copy. If any of dest-start, src-start, or src-end are out of range (taking into account the sizes of the strings and the source and destination regions), the exn:fail:contract exception is raised.

Examples:

> (define s (string #\A #\p #\p #\l #\e))

> (string-copy! s 4 "y")

> (string-copy! s 0 s 3 4)

> s

"lpply"

(string-fill! dest char) → void?

dest : (and/c string? (not/c immutable?))

char : char?

Changes dest so that every position in the string is filled with char.

Examples:

> (define s (string #\A #\p #\p #\l #\e))

> (string-fill! s #\q)

> s

"qqqqq"

(string-append str ...) → string?

str : string?

Returns a new mutable string that is as long as the sum of the given strs’ lengths, and that contains the concatenated characters of the given strs. If no strs are provided, the result is a zero-length string.

Examples:

> (string-append "Apple" "Banana")

"AppleBanana"

(string->list str) → (listof char?)

str : string?

Returns a new list of characters coresponding to the content of str. That is, the length of the list is (string-length (scheme str)), and the sequence of characters of str are in the same sequence in the result list.

Examples:

> (string->list "Apple")

(#\A #\p #\p #\l #\e)

(list->string lst) → string?

lst : (listof char?)

Returns a new mutable string whose content is the list of characters in lst. That is, the length of the string is (length (scheme lst)), and the sequence of characters in lst is in the same sequence in the result string.

Examples:

> (list->string (list #\A #\p #\p #\l #\e))

"Apple"

(build-string n proc) → string?

n : exact-nonnegative-integer?

proc : (exact-nonnegative-integer? . -> . char?)

Creates a string of n characters by applying proc to the integers from 0 to (sub1 n) in order. If str is the resulting string, then (string-ref str i) is the character produced by (proc i).

Examples:

> (build-string 5 (lambda (i) (integer->char (+ i 97))))

"abcde"

3.3.2 String Comparisons

(string=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Returns #t if all of the arguments are equal?.

Examples:

> (string=? "Apple" "apple")

#f

> (string=? "a" "as" "a")

#f

(string<? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Returns #t if the arguments are lexicographically sorted increasing, where individual characters are ordered by char<?, #f otherwise.

Examples:

> (string<? "Apple" "apple")

#t

> (string<? "apple" "Apple")

#f

> (string<? "a" "b" "c")

#t

(string<=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string<?, but checks whether the arguments are nondecreasing.

Examples:

> (string<=? "Apple" "apple")

#t

> (string<=? "apple" "Apple")

#f

> (string<=? "a" "b" "b")

#t

(string>? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string<?, but checks whether the arguments are decreasing.

Examples:

> (string>? "Apple" "apple")

#f

> (string>? "apple" "Apple")

#t

> (string>? "c" "b" "a")

#t

(string>=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string<?, but checks whether the arguments are nonincreasing.

Examples:

> (string>=? "Apple" "apple")

#f

> (string>=? "apple" "Apple")

#t

> (string>=? "c" "b" "b")

#t

(string-ci=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Returns #t if all of the arguments are eqv? after locale-insensitive case-folding via string-foldcase.

Examples:

> (string-ci=? "Apple" "apple")

#t

> (string-ci=? "a" "a" "a")

#t

(string-ci<? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string<?, but checks whether the arguments would be in increasing order if each was first case-folded using string-foldcase (which is locale-insensitive).

Examples:

> (string-ci<? "Apple" "apple")

#f

> (string-ci<? "apple" "banana")

#t

> (string-ci<? "a" "b" "c")

#t

(string-ci<=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string-ci<?, but checks whether the arguments would be nondecreasing after case-folding.

Examples:

> (string-ci<=? "Apple" "apple")

#t

> (string-ci<=? "apple" "Apple")

#t

> (string-ci<=? "a" "b" "b")

#t

(string-ci>? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string-ci<?, but checks whether the arguments would be decreasing after case-folding.

Examples:

> (string-ci>? "Apple" "apple")

#f

> (string-ci>? "banana" "Apple")

#t

> (string-ci>? "c" "b" "a")

#t

(string-ci>=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string-ci<?, but checks whether the arguments would be nonincreasing after case-folding.

Examples:

> (string-ci>=? "Apple" "apple")

#t

> (string-ci>=? "apple" "Apple")

#t

> (string-ci>=? "c" "b" "b")

#t

3.3.3 String Conversions

(string-upcase str) → string?

str : string?

Returns a string whose characters are the upcase conversion of the characters in str. The conversion uses Unicode’s locale-independent conversion rules that map code-point sequences to code-point sequences (instead of simply mapping a 1-to-1 function on code points over the string), so the string produced by the conversion can be longer than the input string.

Examples:

> (string-upcase "abc!")

"ABC!"

> (string-upcase "Straße")

"STRASSE"

(string-downcase string) → string?

string : string?

Like string-upcase, but the downcase conversion.

Examples:

> (string-downcase "aBC!")

"abc!"

> (string-downcase "Straße")

"straße"

> (string-downcase "ΚΑΟΣ")

"καος"

> (string-downcase "Σ")

"σ"

(string-titlecase string) → string?

string : string?

Like string-upcase, but the titlecase conversion only for the first character in each sequence of cased characters in str (ignoring case-ignorable characters).

Examples:

> (string-titlecase "aBC  twO")

"Abc  Two"

> (string-titlecase "y2k")

"Y2K"

> (string-titlecase "main straße")

"Main Straße"

> (string-titlecase "stra ße")

"Stra Sse"

(string-foldcase string) → string?

string : string?

Like string-upcase, but the case-folding conversion.

Examples:

> (string-foldcase "aBC!")

"abc!"

> (string-foldcase "Straße")

"strasse"

> (string-foldcase "ΚΑΟΣ")

"καοσ"

(string-normalize-nfd string) → string?

string : string?

Returns a string that is the Unicode normalized form D of string. If the given string is already in the corresponding Unicode normal form, the string may be returned directly as the result (instead of a newly allocated string).

(string-normalize-nfkd string) → string?

string : string?

Like string-normalize-nfd, but for normalized form KD.

(string-normalize-nfc string) → string?

string : string?

Like string-normalize-nfd, but for normalized form C.

(string-normalize-nfkc string) → string?

string : string?

Like string-normalize-nfd, but for normalized form KC.

3.3.4 Locale-Specific String Operations

(string-locale=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string=?, but the strings are compared in a locale-specific way, based the value of current-locale. See Encodings and Locales for more information on locales.

(string-locale<? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string<?, but the sort order compares strings in a locale-specific way, based the value of current-locale. In particular, the sort order may not be simply a lexicographic extension of character ordering.

(string-locale>? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string>?, but locale-specific like string-locale<?.

(string-locale-ci=? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string-locale=?, but strings are compared using rules that are both locale-specific and case-insensitive (depending on what “case-insensitive” means for the current locale).

(string-locale-ci<? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string<?, but both locale-sensitive and case-insensitive like string-locale-ci=?.

(string-locale-ci>? str1 str2 ...+) → boolean?

str1 : string?

str2 : string?

Like string>?, but both locale-sensitive and case-insensitive like string-locale-ci=?.

(string-locale-upcase string) → string?

string : string?

Like string-upcase, but using locale-specific case-conversion rules based the value of current-locale.

(string-locale-downcase string) → string?

string : string?

Like string-downcase, but using locale-specific case-conversion rules based the value of current-locale.

3.3.5 Additional String Functions

(require scheme/string)

The bindings documented in this section are provided by the scheme/string and scheme libraries, but not scheme/base.

(string-append* str ... strs) → string?

str : string?

strs : (listof string?)

Like string-append, but the last argument is used as a list of arguments for string-append, so (string-append* str ... strs) is the same as (apply string-append str ... strs). In other words, the relationship between string-append and string-append* is similar to the one between list and list*.

Examples:

> (string-append* "a" "b" '("c" "d"))

"abcd"

> (string-append* (cdr (append* (map (lambda (x) (list ", " x))                                        '("Alpha" "Beta" "Gamma")))))

"Alpha, Beta, Gamma"

(string-join strs sep) → string?

strs : (listof string?)

sep : string?

Appends the strings in strs, inserting sep between each pair of strings in strs.

Examples:

> (string-join '("one" "two" "three" "four") " potato ")

"one potato two potato three potato four"