A string is a sequence of characters. Unlike symbols, which are atomic, strings can be separated into constituent parts.
Procedures covered in this reading:
A string is a sequence of zero or more characters. Most strings can be
named by enclosing the characters they contain between plain double
quotation marks, to produce a string literal: for instance,
"hyperbola" is the nine-character string consisting of the
#\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
#\", 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
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
Scheme provides several basic procedures for working with strings:
predicate determines whether its argument is or is not a string.
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) "aaaaa"
constructs and returns the string
(string ch1 ... chn) 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
do just what you'd expect.
string-length procedure takes any string as argument and
returns the number of characters in that string. For instance, the value
(string-length "parabola") is 8 and the value of
(string-length "a\\b") is 3.
string-ref procedure is used to select the character at a
specified position within a string. Like
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
"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>? are the case-sensitive versions, and
string-ci>? the case-insensitive ones.
(substring str start end)
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.
(substring "hypocycloid" 3 8) returns the
"ocycl" -- the substring that starts after
"hyp" and ends after the eighth character,
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
8 October 1997 [John Stone]
2 October 2000 [Sam Rebelsky]
4 February 2001 [Samuel A. Rebelsky]
7 February 2001 [Samuel A. Rebelsky]
Monday, 26 February 2001 [Samuel A. Rebelsky]
Sunday, 15 September 2002 [Samuel A. Rebelsky]
Monday, 16 September 2002 [Samuel A. Rebelsky]
I usually create these pages
on the fly, which means that I rarely
proofread them and they may contain bad grammar and incorrect details.
It also means that I tend to update them regularly (see the history for
more details). Feel free to contact me with any suggestions for changes.
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