A string is a sequence of zero or more characters. Most strings can be
named by enclosing the characters they contain between double quotation
marks, to produce a string literal: for instance,
"hyperbola" is the nine-character string consisting of the
characters #\h, #\y, #\p,
#\e, #\r, #\b, #\o,
#\l, and #\a, in that order, and ""
is the zero-character string (the null string).
String literals may contain spaces and newline characters; when such
characters are between double quotation marks, they are treated like any
other characters in the string. There is a slight problem when one wants
to put a double quotation mark into a string literal: To indicate that the
double quotation mark is part of the string (rather than marking the end
of
the string), one must place a backslash character immediately in front of
it. For instance, "Say \"hi\"" is the eight-character
string consisting of the characters #\S, #\a,
#\y, #\space, #\",
#\h,
#\i, and #\", in that order. The backslash
before a double quotation mark in a string literal is an escape
character, present only to indicate that the character immediately
following it is part of the string.
This use of the backslash character causes yet another slight problem:
What if one wants to put a backslash into a string? The solution is to
place another backslash character immediately in front of it. For
instance,
"a\\b" is the three-character string consisting of the
characters #\a, #\\, and #\b, in
that order. The first backslash in the string literal is an escape, and
the second is the character that it protects, the one that is part of the
string.
Scheme provides several basic procedures for working with strings:
The string? predicate determines whether its argument is or
is
not a string.
The make-string procedure constructs and returns a string
that
consists of repetitions of a single character. Its first argument
indicates how long the string should be, and the second argument specifies
which character it should be made of. For instance, (make-string 5
#\a) constructs and returns the string "aaaaa".
The string procedure takes any number of characters as
arguments and constructs and returns a string consisting of exactly those
characters. For instance, (string #\H #\i #\!) constructs
and
returns the string "Hi!".
The string-length procedure takes any string as argument and
returns the number of characters in that string. For instance, the value
of (string-length "parabola") is 8 and the value of
(string-length "a\\b") is 3.
The string-ref procedure is used to select the character at a
specified position within a string. Like list-ref,
string-ref presupposes zero-based indexing; the
position is specified by the number of characters that precede it in the
string. (So the first character in the string is at position 0, the
second
at position 1, and so on.) For instance, the value of (string-ref
"ellipse" 4) is #\p -- the character that follows four
other characters and so is at position 4 in zero-based indexing.
Strings can be compared for ``lexicographic order,'' the extension of
alphabetical order that is derived from the collating sequence of the
local
character set. Once more, Scheme provides both case-sensitive and
case-insensitive versions of these predicates: string<?,
string<=?, string=?,
string>=?,
and string>? are the case-sensitive versions, and
string-ci<?, string-ci<=?,
string-ci=?, string-ci>=?, and
string-ci>? the case-insensitive ones.
The substring procedure takes three arguments. The first is
a
string and the second and third are non-negative integers not exceeding
the
length of that string. Substring returns the part of its
first argument that starts after the number of characters specified by the
second argument and ends after the number of characters specified by the
third argument. For instance: (substring "hypocycloid" 3
8) returns the substring "ocycl" -- the substring that
starts after the initial "hyp" and ends after the eighth
character, the l.
The string-append procedure takes any number of strings as
arguments and returns a string formed by concatenating those arguments.
For instance, the value of (string-append "al" "fal" "fa") is
"alfalfa".
This document is available on the World Wide Web as
http://www.cs.grinnell.edu/~gum/courses/151/readings/strings.xhtml
created March 5, 1997
last revised August 11, 2001
Henry Walker (walker@math.grin.edu) and John David Stone (stone@cs.grinnell.edu) and Ben Gum (gum@cs.grinnell.edu)