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.
See section Putting Keyboard Events in Strings, for special considerations for strings of keyboard character events.
format: Emacs's analog of printf.
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 not terminated by a distinguished character code. (By contrast, strings in C are terminated by a character with ASCII code 0.) This means that any character, including the null character (ASCII code 0), is a valid element of a string.
Since strings are considered arrays, you can operate on them with the
general array functions. (See section Sequences, Arrays, and Vectors.) For
example, you can access or change individual characters in a string
using the functions aref and aset (see section Functions that Operate on Arrays).
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 ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. See section 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. See section Text Properties.
See section Text, for information about functions that display strings or copy them into buffers. See section Character Type, and section String Type, for information about the syntax of characters and strings.
For more information about general sequence and array predicates, see section Sequences, Arrays, and Vectors, and section Arrays.
t if object is a string, nil
otherwise.
t if object is a string or a
character (i.e., an integer), nil otherwise.
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.
(make-string 5 ?x)
=> "xxxxx"
(make-string 0 ?x)
=> ""
Other functions to compare with this one include char-to-string
(see section Conversion of Characters and Strings), make-vector (see section Vectors), and
make-list (see section Building Cons Cells and Lists).
(substring "abcdefg" 0 3)
=> "abc"
Here the index for `a' is 0, the index for `b' is 1, and the
index for `c' is 2. Thus, three letters, `abc', are copied
from the string "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 -1 signifies the index of the last character of the string. For example:
(substring "abcdefg" -3 -1)
=> "ef"
In this example, the index for `e' is -3, the index for `f' is -2, and the index for `g' is -1. Therefore, `e' and `f' are included, and `g' is excluded.
When nil is used as an index, it stands for the length of the
string. Thus,
(substring "abcdefg" -3 nil)
=> "efg"
Omitting the argument end is equivalent to specifying nil.
It follows that (substring string 0) returns a copy of all
of string.
(substring "abcdefg" 0)
=> "abcdefg"
But we recommend copy-sequence for this purpose (see section Sequences).
A wrong-type-argument error is signaled if either start or
end is not an integer or nil. An args-out-of-range
error is signaled if start indicates a character following
end, or if either integer is out of range for string.
Contrast this function with buffer-substring (see section Examining 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.
concat receives no arguments, it returns an empty string.
(concat "abc" "-def")
=> "abc-def"
(concat "abc" (list 120 (+ 256 121)) [122])
=> "abcxyz"
;; nil is an empty sequence.
(concat "abc" nil "-def")
=> "abc-def"
(concat "The " "quick brown " "fox.")
=> "The quick brown fox."
(concat)
=> ""
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 concat function always constructs a new string that is
not 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. This special case exists for
compatibility with Mocklisp, and we don't recommend you take advantage
of it. If you want to convert an integer to digits in this way, use
format (see section Formatting Strings) or number-to-string
(see section Conversion of Characters and Strings).
(concat 137)
=> "137"
(concat 54 321)
=> "54321"
For information about other concatenation functions, see the
description of mapconcat in section Mapping Functions,
vconcat in section Vectors, and append in section Building Cons Cells and Lists.
t if the arguments represent the same
character, nil otherwise. This function ignores differences
in case if case-fold-search is non-nil.
(char-equal ?x ?x)
=> t
(char-to-string (+ 256 ?x))
=> "x"
(char-equal ?x (+ 256 ?x))
=> t
t if the characters of the two strings
match exactly; case is significant.
(string= "abc" "abc")
=> t
(string= "abc" "ABC")
=> nil
(string= "ab" "ABC")
=> nil
string-equal is another name for string=.
t. If the lesser character is the one from
string2, then string1 is greater, and this function returns
nil. If the two strings match entirely, the value is nil.
Pairs of characters are compared by their ASCII codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; numbers and many punctuation characters have a lower numeric value than upper case letters.
(string< "abc" "abd")
=> t
(string< "abd" "abc")
=> nil
(string< "123" "abc")
=> t
When the strings have different lengths, and they match up to the
length of string1, then the result is t. If they match up
to the length of string2, the result is nil. A string of
no characters is less than any other string.
(string< "" "abc")
=> t
(string< "ab" "abc")
=> t
(string< "abc" "")
=> nil
(string< "abc" "ab")
=> nil
(string< "" "")
=> nil
string-lessp is another name for string<.
See also compare-buffer-substrings in section Comparing Text, for
a way to compare text in buffers. The function string-match,
which matches a regular expression against a string, can be used
for a kind of string comparison; see section Regular Expression Searching.
This section describes functions for conversions between characters,
strings and integers. format and prin1-to-string
(see section Output Functions) can also convert Lisp objects into strings.
read-from-string (see section Input Functions) can "convert" a
string representation of a Lisp object into an object.
See section Documentation, for functions that produce textual descriptions
of text characters and general input events
(single-key-description and text-char-description). These
functions are used primarily for making help messages.
This function is similar to make-string with an integer argument
of 1. (See section Creating Strings.) This conversion can also be done with
format using the `%c' format specification.
(See section Formatting Strings.)
(char-to-string ?x)
=> "x"
(char-to-string (+ 256 ?x))
=> "x"
(make-string 1 ?x)
=> "x"
(string-to-char "ABC")
=> 65
(string-to-char "xyz")
=> 120
(string-to-char "")
=> 0
(string-to-char "\000")
=> 0
This function may be eliminated in the future if it does not seem useful enough to retain.
(number-to-string 256)
=> "256"
(number-to-string -23)
=> "-23"
(number-to-string -23.5)
=> "-23.5"
int-to-string is a semi-obsolete alias for this function.
See also the function format in section Formatting Strings.
(string-to-number "256")
=> 256
(string-to-number "25 is a perfect square.")
=> 25
(string-to-number "X256")
=> 0
(string-to-number "-4.5")
=> -4.5
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 format string.
Formatting is often useful for computing messages to be displayed. In
fact, the functions message and error provide the same
formatting feature described here; they differ from format only
in how they use the result of formatting.
A format specification is a sequence of characters beginning with a
`%'. Thus, if there is a `%d' in string, the
format function replaces it with the printed representation of
one of the values to be formatted (one of the arguments objects).
For example:
(format "The value of fill-column is %d." fill-column)
=> "The value of fill-column is 72."
If string contains more than one format specification, the format specifications correspond with successive values from objects. Thus, the first format specification in 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:
(format "%%
%d" 30) returns "% 30".
Any other format character results in an `Invalid format operation' error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name))
=> "The name of this buffer is strings.texi."
(format "The buffer object prints as %s." (current-buffer))
=> "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)
=> "The octal value of 18 is 22,
and the hex value is 12."
All the specification characters allow an optional numeric prefix between the `%' 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.
(format "%06d is padded on the left with zeros" 123)
=> "000123 is padded on the left with zeros"
(format "%-6d is padded on the right" 123)
=> "123 is padded on the right"
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, `%7s' specifies a minimum width
of 7. In the first case, the string inserted in place of `%7s' has
only 3 letters, so 4 blank spaces are inserted for padding. In the
second case, the string "specification" is 13 letters wide but is
not truncated. In the third case, the padding is on the right.
(format "The word `%7s' actually has %d letters in it."
"foo" (length "foo"))
=> "The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification" (length "specification"))
=> "The word `specification' actually has 13 letters in it."
(format "The word `%-7s' actually has %d letters in it."
"foo" (length "foo"))
=> "The word `foo ' actually has 3 letters in it."
The character case functions change the case of single characters or of the contents of strings. The functions convert only alphabetic characters (the letters `A' through `Z' and `a' through `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 `X' and `x' which have ASCII codes 88 and 120 respectively.
When the argument to 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
downcase is a character, 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.
(downcase "The cat in the hat")
=> "the cat in the hat"
(downcase ?X)
=> 120
When the argument to 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 upcase is a character, 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.
(upcase "The cat in the hat")
=> "THE CAT IN THE HAT"
(upcase ?x)
=> 88
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 (See section Table of Syntax Classes).
When the argument to capitalize is a character, capitalize
has the same result as upcase.
(capitalize "The cat in the hat")
=> "The Cat In The Hat"
(capitalize "THE 77TH-HATTED CAT")
=> "The 77th-Hatted Cat"
(capitalize ?x)
=> 88
You can customize case conversion by installing a special 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 (see section Case Changes). You need a case table if you are using a language which has letters other than the standard ASCII letters.
A case table is a list of this form:
(downcase upcase canonicalize equivalences)
where each element is either nil or a string of length 256. The
element downcase says how to map each character to its lower-case
equivalent. The element upcase maps each character to its
upper-case equivalent. If lower and upper case characters are in
one-to-one correspondence, use nil for upcase; then Emacs
deduces the upcase table from 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 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 equivalences is a map that cyclicly permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map `a' into `A' and `A' into `a', and likewise for each set of equivalent characters.)
When you construct a case table, you can provide nil for
canonicalize; then Emacs fills in this string from upcase
and downcase. You can also provide nil for
equivalences; then Emacs fills in this string from
canonicalize. In a case table that is actually in use, those
components are non-nil. Do not try to specify equivalences
without also specifying canonicalize.
Each buffer has a case table. Emacs also has a 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:
nil if object is a valid case
table.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify a string downcase-table provided as an argument; this should be a string to be used as the downcase part of a case table. They also modify the standard syntax table. See section Syntax Tables.
You can load the library `iso-syntax' to set up the standard syntax table and define a case table for the 256-bit ISO Latin 1 character set.