@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2003
-@c Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2002, 2003,
+@c 2004, 2005, 2006 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/strings
@node Strings and Characters, Lists, Numbers, Top
The length of a string (like any array) is fixed, and cannot be
altered once the string exists. Strings in Lisp are @emph{not}
terminated by a distinguished character code. (By contrast, strings in
-C are terminated by a character with @sc{ascii} code 0.)
+C are terminated by a character with @acronym{ASCII} code 0.)
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions.
change individual characters in a string using the functions @code{aref}
and @code{aset} (@pxref{Array Functions}).
- There are two text representations for non-@sc{ascii} characters in
+ There are two text representations for non-@acronym{ASCII} characters in
Emacs strings (and in buffers): unibyte and multibyte (@pxref{Text
-Representations}). An @sc{ascii} character always occupies one byte in a
-string; in fact, when a string is all @sc{ascii}, there is no real
+Representations}). An @acronym{ASCII} character always occupies one byte in a
+string; in fact, when a string is all @acronym{ASCII}, there is no real
difference between the unibyte and multibyte representations.
For most Lisp programming, you don't need to be concerned with these two
representations.
codes in the range 128 to 255.
Strings cannot hold characters that have the hyper, super or alt
-modifiers; they can hold @sc{ascii} control characters, but no other
-control characters. They do not distinguish case in @sc{ascii} control
+modifiers; they can hold @acronym{ASCII} control characters, but no other
+control characters. They do not distinguish case in @acronym{ASCII} control
characters. If you want to store such characters in a sequence, such as
a key sequence, you must use a vector instead of a string.
@xref{Character Type}, for more information about the representation of meta
and other modifiers for keyboard input characters.
Strings are useful for holding regular expressions. You can also
-match regular expressions against strings (@pxref{Regexp Search}). The
-functions @code{match-string} (@pxref{Simple Match Data}) and
-@code{replace-match} (@pxref{Replacing Match}) are useful for
-decomposing and modifying strings based on regular expression matching.
+match regular expressions against strings with @code{string-match}
+(@pxref{Regexp Search}). The functions @code{match-string}
+(@pxref{Simple Match Data}) and @code{replace-match} (@pxref{Replacing
+Match}) are useful for decomposing and modifying strings after
+matching regular expressions against them.
Like a buffer, a string can contain text properties for the characters
in it, as well as the characters themselves. @xref{Text Properties}.
otherwise.
@end defun
+@defun string-or-null-p object
+This function returns @code{t} if @var{object} is a string or nil,
+@code{nil} otherwise.
+@end defun
+
@defun char-or-string-p object
This function returns @code{t} if @var{object} is a string or a
character (i.e., an integer), @code{nil} otherwise.
whenever there are two consecutive matches for @var{separators}, or a
match is adjacent to the beginning or end of @var{string}. If
@var{omit-nulls} is @code{t}, these null strings are omitted from the
-result list.
+result.
If @var{separators} is @code{nil} (or omitted),
the default is the value of @code{split-string-default-separators}.
@result{} ("two" "words")
@end example
-The result is not @samp{("" "two" "words" "")}, which would rarely be
-useful. If you need such a result, use an explict value for
+The result is not @code{("" "two" "words" "")}, which would rarely be
+useful. If you need such a result, use an explicit value for
@var{separators}:
@example
-(split-string " two words " split-string-default-separators)
+(split-string " two words "
+ split-string-default-separators)
@result{} ("" "two" "words" "")
@end example
@end defun
@defvar split-string-default-separators
-The default value of @var{separators} for @code{split-string}, initially
-@w{@samp{"[ \f\t\n\r\v]+"}}.
+The default value of @var{separators} for @code{split-string}. Its
+usual value is @w{@code{"[ \f\t\n\r\v]+"}}.
@end defvar
@node Modifying Strings
an error if @var{obj} doesn't fit within @var{string}'s actual length,
or if any new character requires a different number of bytes from the
character currently present at that point in @var{string}.
+@end defun
+
+ To clear out a string that contained a password, use
+@code{clear-string}:
+
+@defun clear-string string
+This clears the contents of @var{string} to zeros.
+It may also change @var{string}'s length and convert it to
+a unibyte string.
@end defun
@need 2000
strings. When @code{equal} (@pxref{Equality Predicates}) compares two
strings, it uses @code{string=}.
-If the strings contain non-@sc{ascii} characters, and one is unibyte
-while the other is multibyte, then they cannot be equal. @xref{Text
+For technical reasons, a unibyte and a multibyte string are
+@code{equal} if and only if they contain the same sequence of
+character codes and all these codes are either in the range 0 through
+127 (@acronym{ASCII}) or 160 through 255 (@code{eight-bit-graphic}).
+However, when a unibyte string gets converted to a multibyte string,
+all characters with codes in the range 160 through 255 get converted
+to characters with higher codes, whereas @acronym{ASCII} characters
+remain unchanged. Thus, a unibyte string and its conversion to
+multibyte are only @code{equal} if the string is all @acronym{ASCII}.
+Character codes 160 through 255 are not entirely proper in multibyte
+text, even though they can occur. As a consequence, the situation
+where a unibyte and a multibyte string are @code{equal} without both
+being all @acronym{ASCII} is a technical oddity that very few Emacs
+Lisp programmers ever get confronted with. @xref{Text
Representations}.
@end defun
Pairs of characters are compared according to their character codes.
Keep in mind that lower case letters have higher numeric values in the
-@sc{ascii} character set than their upper case counterparts; digits and
+@acronym{ASCII} character set than their upper case counterparts; digits and
many punctuation characters have a lower numeric value than upper case
-letters. An @sc{ascii} character is less than any non-@sc{ascii}
-character; a unibyte non-@sc{ascii} character is always less than any
-multibyte non-@sc{ascii} character (@pxref{Text Representations}).
+letters. An @acronym{ASCII} character is less than any non-@acronym{ASCII}
+character; a unibyte non-@acronym{ASCII} character is always less than any
+multibyte non-@acronym{ASCII} character (@pxref{Text Representations}).
@example
@group
the string).
The strings are both converted to multibyte for the comparison
-(@pxref{Text Representations}) so that a unibyte string can be equal to
-a multibyte string. If @var{ignore-case} is non-@code{nil}, then case
-is ignored, so that upper case letters can be equal to lower case letters.
+(@pxref{Text Representations}) so that a unibyte string and its
+conversion to multibyte are always regarded as equal. If
+@var{ignore-case} is non-@code{nil}, then case is ignored, so that
+upper case letters can be equal to lower case letters.
If the specified portions of the two strings match, the value is
@code{t}. Otherwise, the value is an integer which indicates how many
portion) is less.
@end defun
-@defun assoc-ignore-case key alist
+@defun assoc-string key alist &optional case-fold
This function works like @code{assoc}, except that @var{key} must be a
-string, and comparison is done using @code{compare-strings}, ignoring
-case differences. @xref{Association Lists}.
+string, and comparison is done using @code{compare-strings}. If
+@var{case-fold} is non-@code{nil}, it ignores case differences.
+Unlike @code{assoc}, this function can also match elements of the alist
+that are strings rather than conses. In particular, @var{alist} can
+be a list of strings rather than an actual alist.
+@xref{Association Lists}.
@end defun
-@defun assoc-ignore-representation key alist
-This function works like @code{assoc}, except that @var{key} must be a
-string, and comparison is done using @code{compare-strings}.
-Case differences are significant.
-@end defun
-
- See also @code{compare-buffer-substrings} in @ref{Comparing Text}, for
-a way to compare text in buffers. The function @code{string-match},
-which matches a regular expression against a string, can be used
-for a kind of string comparison; see @ref{Regexp Search}.
+ See also the @code{compare-buffer-substrings} function in
+@ref{Comparing Text}, for a way to compare text in buffers. The
+function @code{string-match}, which matches a regular expression
+against a string, can be used for a kind of string comparison; see
+@ref{Regexp Search}.
@node String Conversion
@comment node-name, next, previous, up
@cindex conversion of strings
This section describes functions for conversions between characters,
-strings and integers. @code{format} and @code{prin1-to-string}
+strings and integers. @code{format} (@pxref{Formatting Strings})
+and @code{prin1-to-string}
(@pxref{Output Functions}) can also convert Lisp objects into strings.
@code{read-from-string} (@pxref{Input Functions}) can ``convert'' a
string representation of a Lisp object into an object. The functions
@xref{Documentation}, for functions that produce textual descriptions
of text characters and general input events
(@code{single-key-description} and @code{text-char-description}). These
-functions are used primarily for making help messages.
+are used primarily for making help messages.
@defun char-to-string character
@cindex character to string
@cindex string to character
This function returns the first character in @var{string}. If the
string is empty, the function returns 0. The value is also 0 when the
-first character of @var{string} is the null character, @sc{ascii} code
+first character of @var{string} is the null character, @acronym{ASCII} code
0.
@example
arguments @var{objects} are the computed values to be formatted.
The characters in @var{string}, other than the format specifications,
-are copied directly into the output; starting in Emacs 21, if they have
-text properties, these are copied into the output also.
+are copied directly into the output, including their text properties,
+if any.
@end defun
@cindex @samp{%} in format
@end group
@end example
+ Since @code{format} interprets @samp{%} characters as format
+specifications, you should @emph{never} pass an arbitrary string as
+the first argument. This is particularly true when the string is
+generated by some Lisp code. Unless the string is @emph{known} to
+never include any @samp{%} characters, pass @code{"%s"}, described
+below, as the first argument, and the string as the second, like this:
+
+@example
+ (format "%s" @var{arbitrary-string})
+@end example
+
If @var{string} contains more than one format specification, the
format specifications correspond to successive values from
@var{objects}. Thus, the first format specification in @var{string}
uses the first such value, the second format specification uses the
second such value, and so on. Any extra format specifications (those
-for which there are no corresponding values) cause unpredictable
-behavior. Any extra values to be formatted are ignored.
+for which there are no corresponding values) cause an error. Any
+extra values to be formatted are ignored.
Certain format specifications require values of particular types. If
you supply a value that doesn't fit the requirements, an error is
by their contents alone, with no @samp{"} characters, and symbols appear
without @samp{\} characters.
-Starting in Emacs 21, if the object is a string, its text properties are
+If the object is a string, its text properties are
copied into the output. The text properties of the @samp{%s} itself
are also copied, but those of the object take priority.
@end group
@end example
-@cindex numeric prefix
@cindex field width
@cindex padding
- All the specification characters allow an optional numeric prefix
-between the @samp{%} and the character. The optional numeric prefix
-defines the minimum width for the object. If the printed
-representation of the object contains fewer characters than this, then
-it is padded. The padding is on the left if the prefix is positive
-(or starts with zero) and on the right if the prefix is negative. The
-padding character is normally a space, but if the numeric prefix
-starts with a zero, zeros are used for padding. Some of these
-conventions are ignored for specification characters for which they do
-not make sense. That is, %s, %S and %c accept a numeric prefix
-starting with 0, but still pad with @emph{spaces} on the left. Also,
-%% accepts a numeric prefix, but ignores it. Here are some examples
-of padding:
+ All the specification characters allow an optional ``width,'' which
+is a digit-string between the @samp{%} and the character. If the
+printed representation of the object contains fewer characters than
+this width, then it is padded. The padding is on the left if the
+width is positive (or starts with zero) and on the right if the
+width is negative. The padding character is normally a space, but if
+the width starts with a zero, zeros are used for padding. Some of
+these conventions are ignored for specification characters for which
+they do not make sense. That is, @samp{%s}, @samp{%S} and @samp{%c}
+accept a width starting with 0, but still pad with @emph{spaces} on
+the left. Also, @samp{%%} accepts a width, but ignores it. Here are
+some examples of padding:
@example
(format "%06d is padded on the left with zeros" 123)
@result{} "123 is padded on the right"
@end example
- @code{format} never truncates an object's printed representation, no
-matter what width you specify. Thus, you can use a numeric prefix to
-specify a minimum spacing between columns with no risk of losing
-information.
+If the width is too small, @code{format} does not truncate the
+object's printed representation. Thus, you can use a width to specify
+a minimum spacing between columns with no risk of losing information.
In the following three examples, @samp{%7s} specifies a minimum width
of 7. In the first case, the string inserted in place of @samp{%7s} has
@end group
@end smallexample
+@cindex precision in format specifications
+ All the specification characters allow an optional ``precision''
+before the character (after the width, if present). The precision is
+a decimal-point @samp{.} followed by a digit-string. For the
+floating-point specifications (@samp{%e}, @samp{%f}, @samp{%g}), the
+precision specifies how many decimal places to show; if zero, the
+decimal-point itself is also omitted. For @samp{%s} and @samp{%S},
+the precision truncates the string to the given width, so
+@samp{%.3s} shows only the first three characters of the
+representation for @var{object}. Precision is ignored for other
+specification characters.
+
+@cindex flags in format specifications
+Immediately after the @samp{%} and before the optional width and
+precision, you can put certain ``flag'' characters.
+
+A space character inserts a space for positive numbers (otherwise
+nothing is inserted for positive numbers). This flag is ignored
+except for @samp{%d}, @samp{%e}, @samp{%f}, @samp{%g}.
+
+The flag @samp{#} indicates ``alternate form.'' For @samp{%o} it
+ensures that the result begins with a 0. For @samp{%x} and @samp{%X}
+the result is prefixed with @samp{0x} or @samp{0X}. For @samp{%e},
+@samp{%f}, and @samp{%g} a decimal point is always shown even if the
+precision is zero.
+
@node Case Conversion
@comment node-name, next, previous, up
@section Case Conversion in Lisp
The character case functions change the case of single characters or
of the contents of strings. The functions normally convert only
alphabetic characters (the letters @samp{A} through @samp{Z} and
-@samp{a} through @samp{z}, as well as non-@sc{ascii} letters); other
+@samp{a} through @samp{z}, as well as non-@acronym{ASCII} letters); other
characters are not altered. You can specify a different case
conversion mapping by specifying a case table (@pxref{Case Tables}).
arguments.
The examples below use the characters @samp{X} and @samp{x} which have
-@sc{ascii} codes 88 and 120 respectively.
+@acronym{ASCII} codes 88 and 120 respectively.
@defun downcase string-or-char
This function converts a character or a string to lower case.
canonical equivalent character (which should be either @samp{a} for both
of them, or @samp{A} for both of them).
- The extra table @var{equivalences} is a map that cyclicly permutes
+ The extra table @var{equivalences} is a map that cyclically permutes
each equivalence class (of characters with the same canonical
-equivalent). (For ordinary @sc{ascii}, this would map @samp{a} into
+equivalent). (For ordinary @acronym{ASCII}, this would map @samp{a} into
@samp{A} and @samp{A} into @samp{a}, and likewise for each set of
equivalent characters.)
@end defun
The following three functions are convenient subroutines for packages
-that define non-@sc{ascii} character sets. They modify the specified
+that define non-@acronym{ASCII} character sets. They modify the specified
case table @var{case-table}; they also modify the standard syntax table.
@xref{Syntax Tables}. Normally you would use these functions to change
the standard case table.