emacs/lispref/strings.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/strings
@node Strings and Characters, Lists, Numbers, Top
@comment  node-name,  next,  previous,  up
@chapter Strings and Characters
@cindex strings
@cindex character arrays
@cindex characters
@cindex bytes

  A string in Emacs Lisp is an array that contains an ordered sequence
of characters.  Strings are used as names of symbols, buffers, and
files, to send messages to users, to hold text being copied between
buffers, and for many other purposes.  Because strings are so important,
Emacs Lisp has many functions expressly for manipulating them.  Emacs
Lisp programs use strings more often than individual characters.

  @xref{Strings of Events}, for special considerations for strings of
keyboard character events.

@menu
* Basics: String Basics.      Basic properties of strings and characters.
* Predicates for Strings::    Testing whether an object is a string or char.
* Creating Strings::          Functions to allocate new strings.
* Text Comparison::           Comparing characters or strings.
* String Conversion::         Converting characters or strings and vice versa.
* Formatting Strings::        @code{format}: Emacs's analog of @code{printf}.
* Character Case::            Case conversion functions.
* Case Table::		      Customizing case conversion.
@end menu

@node String Basics
@section String and Character Basics

  Strings in Emacs Lisp are arrays that contain an ordered sequence of
characters.  Characters are represented in Emacs Lisp as integers;
whether an integer was intended as a character or not is determined only
by how it is used.  Thus, strings really contain integers.

  The length of a string (like any array) is fixed and independent of
the string contents, and cannot be altered.  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.)
This means that any character, including the null character (@sc{ASCII}
code 0), is a valid element of a string.@refill

  Since strings are considered arrays, you can operate on them with the
general array functions.  (@xref{Sequences Arrays Vectors}.)  For
example, you can access or change individual characters in a string
using the functions @code{aref} and @code{aset} (@pxref{Array
Functions}).

  Each character in a string is stored in a single byte.  Therefore,
numbers not in the range 0 to 255 are truncated when stored into a
string.  This means that a string takes up much less memory than a
vector of the same length.

  Sometimes key sequences are represented as strings.  When a string is
a key sequence, string elements in the range 128 to 255 represent meta
characters (which are extremely large integers) rather than keyboard
events 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
characters.  @xref{Character Type}, for more information about
representation of meta and other modifiers for keyboard input
characters.

  Like a buffer, a string can contain text properties for the characters
in it, as well as the characters themselves.  @xref{Text Properties}.

  @xref{Text}, for information about functions that display strings or
copy them into buffers.  @xref{Character Type}, and @ref{String Type},
for information about the syntax of characters and strings.

@node Predicates for Strings
@section The Predicates for Strings

For more information about general sequence and array predicates,
see @ref{Sequences Arrays Vectors}, and @ref{Arrays}.

@defun stringp object
  This function returns @code{t} if @var{object} is a string, @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.
@end defun

@node Creating Strings
@section Creating Strings

  The following functions create strings, either from scratch, or by
putting strings together, or by taking them apart.

@defun make-string count character
  This function returns a string made up of @var{count} repetitions of
@var{character}.  If @var{count} is negative, an error is signaled.

@example
(make-string 5 ?x)
     @result{} "xxxxx"
(make-string 0 ?x)
     @result{} ""
@end example

  Other functions to compare with this one include @code{char-to-string}
(@pxref{String Conversion}), @code{make-vector} (@pxref{Vectors}), and
@code{make-list} (@pxref{Building Lists}).
@end defun

@defun substring string start &optional end
  This function returns a new string which consists of those characters
from @var{string} in the range from (and including) the character at the
index @var{start} up to (but excluding) the character at the index
@var{end}.  The first character is at index zero.

@example
@group
(substring "abcdefg" 0 3)
     @result{} "abc"
@end group
@end example

@noindent
Here the index for @samp{a} is 0, the index for @samp{b} is 1, and the
index for @samp{c} is 2.  Thus, three letters, @samp{abc}, are copied
from the string @code{"abcdefg"}.  The index 3 marks the character
position up to which the substring is copied.  The character whose index
is 3 is actually the fourth character in the string.

A negative number counts from the end of the string, so that @minus{}1
signifies the index of the last character of the string.  For example: 

@example
@group
(substring "abcdefg" -3 -1)
     @result{} "ef"
@end group
@end example

@noindent
In this example, the index for @samp{e} is @minus{}3, the index for
@samp{f} is @minus{}2, and the index for @samp{g} is @minus{}1.
Therefore, @samp{e} and @samp{f} are included, and @samp{g} is excluded.

When @code{nil} is used as an index, it stands for the length of the
string.  Thus,

@example
@group
(substring "abcdefg" -3 nil)
     @result{} "efg"
@end group
@end example

Omitting the argument @var{end} is equivalent to specifying @code{nil}.
It follows that @code{(substring @var{string} 0)} returns a copy of all
of @var{string}.

@example
@group
(substring "abcdefg" 0)
     @result{} "abcdefg"
@end group
@end example

@noindent
But we recommend @code{copy-sequence} for this purpose (@pxref{Sequence
Functions}).

A @code{wrong-type-argument} error is signaled if either @var{start} or
@var{end} is not an integer or @code{nil}.  An @code{args-out-of-range}
error is signaled if @var{start} indicates a character following
@var{end}, or if either integer is out of range for @var{string}.

Contrast this function with @code{buffer-substring} (@pxref{Buffer
Contents}), which returns a string containing a portion of the text in
the current buffer.  The beginning of a string is at index 0, but the
beginning of a buffer is at index 1.
@end defun

@defun concat &rest sequences
@cindex copying strings
@cindex concatenating strings
This function returns a new string consisting of the characters in the
arguments passed to it.  The arguments may be strings, lists of numbers,
or vectors of numbers; they are not themselves changed.  If
@code{concat} receives no arguments, it returns an empty string.

@example
(concat "abc" "-def")
     @result{} "abc-def"
(concat "abc" (list 120 (+ 256 121)) [122])
     @result{} "abcxyz"
;; @r{@code{nil} is an empty sequence.}
(concat "abc" nil "-def")
     @result{} "abc-def"
(concat "The " "quick brown " "fox.")
     @result{} "The quick brown fox."
(concat)
     @result{} ""
@end example

@noindent
The second example above shows how characters stored in strings are
taken modulo 256.  In other words, each character in the string is
stored in one byte.

The @code{concat} function always constructs a new string that is
not @code{eq} to any existing string.

When an argument is an integer (not a sequence of integers), it is
converted to a string of digits making up the decimal printed
representation of the integer.  @strong{Don't use this feature; we plan
to eliminate it.  If you already use this feature, change your programs
now!}  The proper way to convert an integer to a decimal number in this
way is with @code{format} (@pxref{Formatting Strings}) or
@code{number-to-string} (@pxref{String Conversion}).

@example
@group
(concat 137)
     @result{} "137"
(concat 54 321)
     @result{} "54321"
@end group
@end example

For information about other concatenation functions, see the
description of @code{mapconcat} in @ref{Mapping Functions},
@code{vconcat} in @ref{Vectors}, and @code{append} in @ref{Building
Lists}.
@end defun

@node Text Comparison
@section Comparison of Characters and Strings
@cindex string equality

@defun char-equal character1 character2
This function returns @code{t} if the arguments represent the same
character, @code{nil} otherwise.  This function ignores differences
in case if @code{case-fold-search} is non-@code{nil}.

@example
(char-equal ?x ?x)
     @result{} t
(char-to-string (+ 256 ?x))
     @result{} "x"
(char-equal ?x  (+ 256 ?x))
     @result{} t
@end example
@end defun

@defun string= string1 string2
This function returns @code{t} if the characters of the two strings
match exactly; case is significant.

@example
(string= "abc" "abc")
     @result{} t
(string= "abc" "ABC")
     @result{} nil
(string= "ab" "ABC")
     @result{} nil
@end example

The function @code{string=} ignores the text properties of the
two strings.  To compare strings in a way that compares their text
properties also, use @code{equal} (@pxref{Equality Predicates}).
@end defun

@defun string-equal string1 string2
@code{string-equal} is another name for @code{string=}.
@end defun

@cindex lexical comparison
@defun string< string1 string2
@c (findex string< causes problems for permuted index!!)
This function compares two strings a character at a time.  First it
scans both the strings at once to find the first pair of corresponding
characters that do not match.  If the lesser character of those two is
the character from @var{string1}, then @var{string1} is less, and this
function returns @code{t}.  If the lesser character is the one from
@var{string2}, then @var{string1} is greater, and this function returns
@code{nil}.  If the two strings match entirely, the value is @code{nil}.

Pairs of characters are compared by their @sc{ASCII} codes.  Keep in
mind that lower case letters have higher numeric values in the
@sc{ASCII} character set than their upper case counterparts; numbers and
many punctuation characters have a lower numeric value than upper case
letters.

@example
@group
(string< "abc" "abd")
     @result{} t
(string< "abd" "abc")
     @result{} nil
(string< "123" "abc")
     @result{} t
@end group
@end example

When the strings have different lengths, and they match up to the
length of @var{string1}, then the result is @code{t}.  If they match up
to the length of @var{string2}, the result is @code{nil}.  A string of
no characters is less than any other string.

@example
@group
(string< "" "abc")
     @result{} t
(string< "ab" "abc")
     @result{} t
(string< "abc" "")
     @result{} nil
(string< "abc" "ab")
     @result{} nil
(string< "" "")
     @result{} nil                   
@end group
@end example
@end defun

@defun string-lessp string1 string2
@code{string-lessp} is another name for @code{string<}.
@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}.

@node String Conversion
@comment  node-name,  next,  previous,  up
@section Conversion of Characters and Strings
@cindex conversion of strings

  This section describes functions for conversions between characters,
strings and integers.  @code{format} 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.

  @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.

@defun char-to-string character
@cindex character to string
  This function returns a new string with a length of one character.
The value of @var{character}, modulo 256, is used to initialize the
element of the string.

This function is similar to @code{make-string} with an integer argument
of 1.  (@xref{Creating Strings}.)  This conversion can also be done with
@code{format} using the @samp{%c} format specification.
(@xref{Formatting Strings}.)

@example
(char-to-string ?x)
     @result{} "x"
(char-to-string (+ 256 ?x))
     @result{} "x"
(make-string 1 ?x)
     @result{} "x"
@end example
@end defun

@defun string-to-char 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
0.

@example
(string-to-char "ABC")
     @result{} 65
(string-to-char "xyz")
     @result{} 120
(string-to-char "")
     @result{} 0
(string-to-char "\000")
     @result{} 0
@end example

This function may be eliminated in the future if it does not seem useful
enough to retain.
@end defun

@defun number-to-string number
@cindex integer to string
@cindex integer to decimal
This function returns a string consisting of the printed
representation of @var{number}, which may be an integer or a floating
point number.  The value starts with a sign if the argument is
negative.

@example
(number-to-string 256)
     @result{} "256"
(number-to-string -23)
     @result{} "-23"
(number-to-string -23.5)
     @result{} "-23.5"
@end example

@cindex int-to-string
@code{int-to-string} is a semi-obsolete alias for this function.

See also the function @code{format} in @ref{Formatting Strings}.
@end defun

@defun string-to-number string
@cindex string to number
This function returns the numeric value of the characters in
@var{string}, read in base ten.  It skips spaces and tabs at the
beginning of @var{string}, then reads as much of @var{string} as it can
interpret as a number.  (On some systems it ignores other whitespace at
the beginning, not just spaces and tabs.)  If the first character after
the ignored whitespace is not a digit or a minus sign, this function
returns 0.

@example
(string-to-number "256")
     @result{} 256
(string-to-number "25 is a perfect square.")
     @result{} 25
(string-to-number "X256")
     @result{} 0
(string-to-number "-4.5")
     @result{} -4.5
@end example

@findex string-to-int
@code{string-to-int} is an obsolete alias for this function.
@end defun

@node Formatting Strings
@comment  node-name,  next,  previous,  up
@section Formatting Strings
@cindex formatting strings
@cindex strings, formatting them

  @dfn{Formatting} means constructing a string by substitution of
computed values at various places in a constant string.  This string
controls how the other values are printed as well as where they appear;
it is called a @dfn{format string}.

  Formatting is often useful for computing messages to be displayed.  In
fact, the functions @code{message} and @code{error} provide the same
formatting feature described here; they differ from @code{format} only
in how they use the result of formatting.

@defun format string &rest objects
  This function returns a new string that is made by copying
@var{string} and then replacing any format specification 
in the copy with encodings of the corresponding @var{objects}.  The
arguments @var{objects} are the computed values to be formatted.
@end defun

@cindex @samp{%} in format
@cindex format specification
  A format specification is a sequence of characters beginning with a
@samp{%}.  Thus, if there is a @samp{%d} in @var{string}, the
@code{format} function replaces it with the printed representation of
one of the values to be formatted (one of the arguments @var{objects}).
For example:

@example
@group
(format "The value of fill-column is %d." fill-column)
     @result{} "The value of fill-column is 72."
@end group
@end example

  If @var{string} contains more than one format specification, the
format specifications correspond with 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.

  Certain format specifications require values of particular types.
However, no error is signaled if the value actually supplied fails to
have the expected type.  Instead, the output is likely to be
meaningless.

  Here is a table of valid format specifications:

@table @samp
@item %s
Replace the specification with the printed representation of the object,
made without quoting.  Thus, strings are represented by their contents
alone, with no @samp{"} characters, and symbols appear without @samp{\}
characters.

If there is no corresponding object, the empty string is used.

@item %S
Replace the specification with the printed representation of the object,
made with quoting.  Thus, strings are enclosed in @samp{"} characters,
and @samp{\} characters appear where necessary before special characters.

If there is no corresponding object, the empty string is used.

@item %o
@cindex integer to octal
Replace the specification with the base-eight representation of an
integer.

@item %d
Replace the specification with the base-ten representation of an
integer.

@item %x
@cindex integer to hexadecimal
Replace the specification with the base-sixteen representation of an
integer.

@item %c
Replace the specification with the character which is the value given.

@item %e
Replace the specification with the exponential notation for a floating
point number.

@item %f
Replace the specification with the decimal-point notation for a floating
point number.

@item %g
Replace the specification with notation for a floating point number,
using either exponential notation or decimal-point notation whichever
is shorter.

@item %%
A single @samp{%} is placed in the string.  This format specification is
unusual in that it does not use a value.  For example, @code{(format "%%
%d" 30)} returns @code{"% 30"}.
@end table

  Any other format character results in an @samp{Invalid format
operation} error.

  Here are several examples:

@example
@group
(format "The name of this buffer is %s." (buffer-name))
     @result{} "The name of this buffer is strings.texi."

(format "The buffer object prints as %s." (current-buffer))
     @result{} "The buffer object prints as #<buffer strings.texi>."

(format "The octal value of %d is %o, 
         and the hex value is %x." 18 18 18)
     @result{} "The octal value of 18 is 22, 
         and the hex value is 12."
@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.

@example
(format "%06d is padded on the left with zeros" 123)
     @result{} "000123 is padded on the left with zeros"

(format "%-6d is padded on the right" 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.

  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
only 3 letters, so 4 blank spaces are inserted for padding.  In the
second case, the string @code{"specification"} is 13 letters wide but is
not truncated.  In the third case, the padding is on the right.

@smallexample 
@group
(format "The word `%7s' actually has %d letters in it."
        "foo" (length "foo"))
     @result{} "The word `    foo' actually has 3 letters in it."  
@end group

@group
(format "The word `%7s' actually has %d letters in it."
        "specification" (length "specification")) 
     @result{} "The word `specification' actually has 13 letters in it."  
@end group

@group
(format "The word `%-7s' actually has %d letters in it."
        "foo" (length "foo"))
     @result{} "The word `foo    ' actually has 3 letters in it."  
@end group
@end smallexample

@node Character Case
@comment node-name, next, previous, up 
@section Character Case
@cindex upper case 
@cindex lower case 
@cindex character case 

  The character case functions change the case of single characters or
of the contents of strings.  The functions convert only alphabetic
characters (the letters @samp{A} through @samp{Z} and @samp{a} through
@samp{z}); other characters are not altered.  The functions do not
modify the strings that are passed to them as arguments.

  The examples below use the characters @samp{X} and @samp{x} which have
@sc{ASCII} codes 88 and 120 respectively.

@defun downcase string-or-char
This function converts a character or a string to lower case.

When the argument to @code{downcase} is a string, the function creates
and returns a new string in which each letter in the argument that is
upper case is converted to lower case.  When the argument to
@code{downcase} is a character, @code{downcase} returns the
corresponding lower case character.  This value is an integer.  If the
original character is lower case, or is not a letter, then the value
equals the original character.

@example
(downcase "The cat in the hat")
     @result{} "the cat in the hat"

(downcase ?X)
     @result{} 120
@end example
@end defun

@defun upcase string-or-char
This function converts a character or a string to upper case.

When the argument to @code{upcase} is a string, the function creates
and returns a new string in which each letter in the argument that is
lower case is converted to upper case.

When the argument to @code{upcase} is a character, @code{upcase}
returns the corresponding upper case character.  This value is an integer.
If the original character is upper case, or is not a letter, then the
value equals the original character.

@example
(upcase "The cat in the hat")
     @result{} "THE CAT IN THE HAT"

(upcase ?x)
     @result{} 88
@end example
@end defun

@defun capitalize string-or-char
@cindex capitalization
This function capitalizes strings or characters.  If
@var{string-or-char} is a string, the function creates and returns a new
string, whose contents are a copy of @var{string-or-char} in which each
word has been capitalized.  This means that the first character of each
word is converted to upper case, and the rest are converted to lower
case.

The definition of a word is any sequence of consecutive characters that
are assigned to the word constituent syntax class in the current syntax
table (@xref{Syntax Class Table}).

When the argument to @code{capitalize} is a character, @code{capitalize}
has the same result as @code{upcase}.

@example
(capitalize "The cat in the hat")
     @result{} "The Cat In The Hat"

(capitalize "THE 77TH-HATTED CAT")
     @result{} "The 77th-Hatted Cat"

@group
(capitalize ?x)
     @result{} 88
@end group
@end example
@end defun

@node Case Table
@section The Case Table

  You can customize case conversion by installing a special @dfn{case
table}.  A case table specifies the mapping between upper case and lower
case letters.  It affects both the string and character case conversion
functions (see the previous section) and those that apply to text in the
buffer (@pxref{Case Changes}).  You need a case table if you are using a
language which has letters other than the standard @sc{ASCII} letters.

  A case table is a list of this form:

@example
(@var{downcase} @var{upcase} @var{canonicalize} @var{equivalences})
@end example

@noindent
where each element is either @code{nil} or a string of length 256.  The
element @var{downcase} says how to map each character to its lower-case
equivalent.  The element @var{upcase} maps each character to its
upper-case equivalent.  If lower and upper case characters are in
one-to-one correspondence, use @code{nil} for @var{upcase}; then Emacs
deduces the upcase table from @var{downcase}.

  For some languages, upper and lower case letters are not in one-to-one
correspondence.  There may be two different lower case letters with the
same upper case equivalent.  In these cases, you need to specify the
maps for both directions.

  The element @var{canonicalize} maps each character to a canonical
equivalent; any two characters that are related by case-conversion have
the same canonical equivalent character.

  The element @var{equivalences} is a map that cyclicly permutes each
equivalence class (of characters with the same canonical equivalent).
(For ordinary @sc{ASCII}, this would map @samp{a} into @samp{A} and
@samp{A} into @samp{a}, and likewise for each set of equivalent
characters.)

  When you construct a case table, you can provide @code{nil} for
@var{canonicalize}; then Emacs fills in this string from @var{upcase}
and @var{downcase}.  You can also provide @code{nil} for
@var{equivalences}; then Emacs fills in this string from
@var{canonicalize}.  In a case table that is actually in use, those
components are non-@code{nil}.  Do not try to specify @var{equivalences}
without also specifying @var{canonicalize}.

  Each buffer has a case table.  Emacs also has a @dfn{standard case
table} which is copied into each buffer when you create the buffer.
Changing the standard case table doesn't affect any existing buffers.

  Here are the functions for working with case tables:

@defun case-table-p object
This predicate returns non-@code{nil} if @var{object} is a valid case
table.
@end defun

@defun set-standard-case-table table
This function makes @var{table} the standard case table, so that it will
apply to any buffers created subsequently.
@end defun

@defun standard-case-table
This returns the standard case table.
@end defun

@defun current-case-table
This function returns the current buffer's case table.
@end defun

@defun set-case-table table
This sets the current buffer's case table to @var{table}.
@end defun

  The following three functions are convenient subroutines for packages
that define non-@sc{ASCII} character sets.  They modify a string
@var{downcase-table} provided as an argument; this should be a string to
be used as the @var{downcase} part of a case table.  They also modify
the standard syntax table.  @xref{Syntax Tables}.

@defun set-case-syntax-pair uc lc downcase-table
This function specifies a pair of corresponding letters, one upper case
and one lower case.
@end defun

@defun set-case-syntax-delims l r downcase-table
This function makes characters @var{l} and @var{r} a matching pair of
case-invariant delimiters.
@end defun

@defun set-case-syntax char syntax downcase-table
This function makes @var{char} case-invariant, with syntax
@var{syntax}.
@end defun

@deffn Command describe-buffer-case-table
This command displays a description of the contents of the current
buffer's case table.
@end deffn

@cindex ISO Latin 1
@pindex iso-syntax
You can load the library @file{iso-syntax} to set up the standard syntax
table and define a case table for the 8-bit ISO Latin 1 character set.