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Summary:
In this reading, we consider yet another basic data type: Strings.
A string is a sequence of characters. Unlike symbols, which are
atomic, strings can be separated into constituent parts.
Related readings:
Procedures covered in this reading
- String predicates:
string?
- String constructors:
make-string,
string,
string-append
- String destructors:
string-ref,
substring
- String conversion:
list->string,
number->string,
string->list,
string->number
- String analysis:
string-length,
- String comparison:
string<?,
string<=?,
string=?,
string>=?,
string>?,
string-ci<?,
string-ci<=?,
string-ci=?,
string-ci>=?,
string-ci>?
Contents:
As we've just learned in the
reading on characters, characters provide the basic building blocks
of the things we might call texts
. What do we do with characters?
We combine them into strings.
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
characters #\h, #\y, #\p,
#\e, #\r, #\b, #\o,
#\l, and #\a, in that order. Similarly,
"" is the zero-character string (the null string
or the empty 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 similar:
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? val)
predicate determines whether its argument is or is not a string.
The (make-string count char) procedure
constructs and returns a string that consists of count
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, the following code constructs and
returns the string "aaaaa".
> (make-string 5 #\a)
"aaaaa"
The (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 "Hi!". This procedure can be useful
for building strings with quotes. For example, (string #\" #\")
produces "\"\"". (Isn't that ugly?)
The string->list procedure converts a string into a list
of characters. The list->string procedure converts a
list of characters into a string. It is invalid to call
list->string on a non-list or on a list that contains
values other than characters.
> (string->list "Hello")
(#\H #\e #\l #\l #\o)
> (list->string (list #\a #\b #\c))
"abc"
> (list->string (list 'a 'b))
list->string: expects argument of type <list of character>; given (a b)
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 initial character in the string is at position 0, the next
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 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.
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. (If you think of the characters in a string as
being numbered starting at 0, substring takes the characters
from start to end-1.)
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".
The number->string procedure takes any Scheme number as
its argument and returns a string that denotes the number.
> (number->string 23)
"23"
> (number->string 6/5)
6/5"
> (number->string #i6/5)
"1.2"
> (number->string (sqrt -1))
"0+1i"
The string->number procedure provides the inverse
operation. Given a string that represents a number, it returns the
corresponding number. Fascinatingly, unlike other procedures that give up
if you give them inappropriate input, when string->number
is called with a string that does not represent a number, it returns
the value #f (which represents no
or false
).
> (string->number "23")
23
> (string->number "6/5")
1 1/5
> (string->number "3+4i")
3+4i
> (string->number "")
#f
> (string->number "two")
#f
> (string->number "3 + 4i")
#f
;
