Recall that the sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements.
An array is a single primitive object that has a slot for each of its elements. All the elements are accessible in constant time, but the length of an existing array cannot be changed. Strings, vectors, char-tables and bool-vectors are the four types of arrays.
A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements.
The following diagram shows the relationship between these types:
_____________________________________________
| |
| Sequence |
| ______ ________________________________ |
| | | | | |
| | List | | Array | |
| | | | ________ ________ | |
| |______| | | | | | | |
| | | Vector | | String | | |
| | |________| |________| | |
| | ____________ _____________ | |
| | | | | | | |
| | | Char-table | | Bool-vector | | |
| | |____________| |_____________| | |
| |________________________________| |
|_____________________________________________|
The elements of vectors and lists may be any Lisp objects. The elements of strings are all characters.
In Emacs Lisp, a sequence is either a list or an array. The common property of all sequences is that they are ordered collections of elements. This section describes functions that accept any kind of sequence.
t if
object is a list, vector, or string, nil
otherwise.
nil), a
wrong-type-argument error is signaled. See section Accessing Elements of
Lists, for the related function safe-length.
(length '(1 2 3))
=> 3
(length ())
=> 0
(length "foobar")
=> 6
(length [1 2 3])
=> 3
(length (make-bool-vector 5 nil))
=> 5
nil; otherwise, they trigger an
args-out-of-range error.
(elt [1 2 3 4] 2)
=> 3
(elt '(1 2 3 4) 2)
=> 3
;; We use string to show clearly which character elt returns.
(string (elt "1234" 2))
=> "3"
(elt [1 2 3 4] 4)
error--> Args out of range: [1 2 3 4], 4
(elt [1 2 3 4] -1)
error--> Args out of range: [1 2 3 4], -1
This function generalizes aref (see section Functions that Operate on Arrays) and
nth (see section Accessing
Elements of Lists).
Storing a new element into the copy does not affect the original
sequence, and vice versa. However, the elements of the
new sequence are not copies; they are identical (eq)
to the elements of the original. Therefore, changes made within
these elements, as found via the copied sequence, are also visible
in the original sequence.
If the sequence is a string with text properties, the property list in the copy is itself a copy, not shared with the original's property list. However, the actual values of the properties are shared. See section Text Properties.
See also append in section Building Cons Cells and Lists,
concat in section Creating
Strings, and vconcat in section Vectors, for others ways to copy
sequences.
(setq bar '(1 2))
=> (1 2)
(setq x (vector 'foo bar))
=> [foo (1 2)]
(setq y (copy-sequence x))
=> [foo (1 2)]
(eq x y)
=> nil
(equal x y)
=> t
(eq (elt x 1) (elt y 1))
=> t
;; Replacing an element of one sequence.
(aset x 0 'quux)
x => [quux (1 2)]
y => [foo (1 2)]
;; Modifying the inside of a shared element.
(setcar (aref x 1) 69)
x => [quux (69 2)]
y => [foo (69 2)]
An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, an element of a list requires access time that is proportional to the position of the element in the list.
Emacs defines four types of array, all one-dimensional: strings, vectors, bool-vectors and char-tables. A vector is a general array; its elements can be any Lisp objects. A string is a specialized array; its elements must be characters (i.e., integers between 0 and 255). Each type of array has its own read syntax. See section String Type, and section Vector Type.
All four kinds of array share these characteristics:
aref and aset, respectively
(see section Functions that Operate on
Arrays).
When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.
In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:
By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See section Key Sequence Input.
In this section, we describe the functions that accept all types of arrays.
t if object is an array (i.e., a vector, a
string, a bool-vector or a char-table).
(arrayp [a])
=> t
(arrayp "asdf")
=> t
(arrayp (syntax-table)) ;; A char-table.
=> t
(setq primes [2 3 5 7 11 13])
=> [2 3 5 7 11 13]
(aref primes 4)
=> 11
(aref "abcdefg" 1)
=> 98 ; `b' is ASCII code 98.
See also the function elt, in section Sequences.
(setq w [foo bar baz])
=> [foo bar baz]
(aset w 0 'fu)
=> fu
w
=> [fu bar baz]
(setq x "asdfasfd")
=> "asdfasfd"
(aset x 3 ?Z)
=> 90
x
=> "asdZasfd"
If array is a string and object is not a
character, a wrong-type-argument error results. If
array is a string and object is character,
but object does not use the same number of bytes as the
character currently stored in (aref object
index), that is also an error. See section Splitting Characters.
(setq a [a b c d e f g])
=> [a b c d e f g]
(fillarray a 0)
=> [0 0 0 0 0 0 0]
a
=> [0 0 0 0 0 0 0]
(setq s "When in the course")
=> "When in the course"
(fillarray s ?-)
=> "------------------"
If array is a string and object is not a
character, a wrong-type-argument error results.
The general sequence functions copy-sequence and
length are often useful for objects known to be
arrays. See section Sequences.
Arrays in Lisp, like arrays in most languages, are blocks of memory whose elements can be accessed in constant time. A vector is a general-purpose array of specified length; its elements can be any Lisp objects. (By contrast, a string can hold only characters as elements.) Vectors in Emacs are used for obarrays (vectors of symbols), and as part of keymaps (vectors of commands). They are also used internally as part of the representation of a byte-compiled function; if you print such a function, you will see a vector in it.
In Emacs Lisp, the indices of the elements of a vector start from zero and count up from there.
Vectors are printed with square brackets surrounding the
elements. Thus, a vector whose elements are the symbols
a, b and a is printed as
[a b a]. You can write vectors in the same way in Lisp
input.
A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See section Self-Evaluating Forms.
Here are examples illustrating these principles:
(setq avector [1 two '(three) "four" [five]])
=> [1 two (quote (three)) "four" [five]]
(eval avector)
=> [1 two (quote (three)) "four" [five]]
(eq avector (eval avector))
=> t
Here are some functions that relate to vectors:
t if object is a vector.
(vectorp [a])
=> t
(vectorp "asdf")
=> nil
(vector 'foo 23 [bar baz] "rats")
=> [foo 23 [bar baz] "rats"]
(vector)
=> []
(setq sleepy (make-vector 9 'Z))
=> [Z Z Z Z Z Z Z Z Z]
The value is a newly constructed vector that is not
eq to any existing vector.
(setq a (vconcat '(A B C) '(D E F)))
=> [A B C D E F]
(eq a (vconcat a))
=> nil
(vconcat)
=> []
(vconcat [A B C] "aa" '(foo (6 7)))
=> [A B C 97 97 foo (6 7)]
The vconcat function also allows byte-code function
objects as arguments. This is a special feature to make it easy to
access the entire contents of a byte-code function object. See
section Byte-Code Function
Objects.
The vconcat function also allows integers as
arguments. It converts them to strings of digits, making up the
decimal print representation of the integer, and then uses the
strings instead of the original integers. 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 format
(see section Formatting Strings) or
number-to-string (see section Conversion of Characters and
Strings).
For other concatenation functions, see mapconcat in
section Mapping Functions,
concat in section Creating
Strings, and append in section Building Cons Cells and Lists.
The append function provides a way to convert a
vector into a list with the same elements (see section Building Cons Cells and Lists):
(setq avector [1 two (quote (three)) "four" [five]])
=> [1 two (quote (three)) "four" [five]]
(append avector nil)
=> (1 two (quote (three)) "four" [five])
A char-table is much like a vector, except that it is indexed by
character codes. Any valid character code, without modifiers, can
be used as an index in a char-table. You can access a char-table's
elements with aref and aset, as with any
array. In addition, a char-table can have extra slots to
hold additional data not associated with particular character
codes. Char-tables are constants when evaluated.
Each char-table has a
subtype which is a symbol. The subtype has two purposes:
to distinguish char-tables meant for different uses, and to control
the number of extra slots. For example, display tables are
char-tables with display-table as the subtype, and
syntax tables are char-tables with syntax-table as the
subtype. A valid subtype must have a
char-table-extra-slots property which is an integer
between 0 and 10. This integer specifies the number of extra
slots in the char-table.
A char-table can have a
parent. which is another char-table. If it does, then
whenever the char-table specifies nil for a particular
character c, it inherits the value specified in the
parent. In other words, (aref char-table
c) returns the value from the parent of
char-table if char-table itself specifies
nil.
A char-table can also have a
default value. If so, then (aref
char-table c) returns the default
value whenever the char-table does not specify any other
non-nil value.
nil. You cannot alter the subtype of
a char-table after the char-table is created. There is no argument to specify the length of the char-table, because all char-tables have room for any valid character code as an index.
t if
object is a char-table, otherwise nil.
There is no special function to access the default value of a
char-table. To do that, use (char-table-range
char-table nil).
nil
or another char-table.
A char-table can specify an element value for a single character code; it can also specify a value for an entire character set.
nilniltchar-table-range---either a valid character or a
generic character--and the value is (char-table-range
char-table key). Overall, the key-value pairs passed to function describe all the values stored in char-table.
The return value is always nil; to make this
function useful, function should have side effects. For
example, here is how to examine each element of the syntax
table:
(let (accumulator)
(map-char-table
#'(lambda (key value)
(setq accumulator
(cons (list key value) accumulator)))
(syntax-table))
accumulator)
=>
((475008 nil) (474880 nil) (474752 nil) (474624 nil)
... (5 (3)) (4 (3)) (3 (3)) (2 (3)) (1 (3)) (0 (3)))
A bool-vector is much like a vector, except that it stores only
the values t and nil. If you try to store
any non-nil value into an element of the bool-vector,
the effect is to store t there. As with all arrays,
bool-vector indices start from 0, and the length cannot be changed
once the bool-vector is created. Bool-vectors are constants when
evaluated.
There are two special functions for working with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.
t if object
is a bool-vector, and nil otherwise.