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Sequences, Arrays, and Vectors

Recall that the sequence type is the union of three other Lisp types: lists, vectors, and strings. In other words, any list is a sequence, any vector is a sequence, and any string 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 directly containing all its elements. Therefore, all the elements are accessible in constant time. The length of an existing array cannot be changed. Both strings and vectors are 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. Therefore, 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 elements of vectors and lists may be any Lisp objects. The elements of strings are all characters.

The following diagram shows the relationship between these types:

            ___________________________________
           |                                   |
           |          Sequence                 |
           |  ______   ______________________  |
           | |      | |                      | |
           | | List | |         Array        | |
           | |      | |  ________   _______  | |   
           | |______| | |        | |       | | |
           |          | | String | | Vector| | |
           |          | |________| |_______| | |
           |          |______________________| |
           |___________________________________|

The Relationship between Sequences, Arrays, and Vectors

Sequences

In Emacs Lisp, a sequence is either a list, a vector or a string. The common property that all sequences have is that each is an ordered collection of elements. This section describes functions that accept any kind of sequence.

Function: sequencep object

Returns t if object is a list, vector, or string, nil otherwise.

Function: copy-sequence sequence

Returns a copy of sequence. The copy is the same type of object as the original sequence, and it has the same elements in the same order.

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)]

Function: length sequence

Returns the number of elements in sequence. If sequence is a cons cell that is not a list (because the final CDR is not nil), a wrong-type-argument error is signaled.

(length '(1 2 3))
    => 3
(length ())
    => 0
(length "foobar")
    => 6
(length [1 2 3])
    => 3

Function: elt sequence index

This function returns the element of sequence indexed by index. Legitimate values of index are integers ranging from 0 up to one less than the length of sequence. If sequence is a list, then out-of-range values of index return nil; otherwise, they produce an args-out-of-range error.

(elt [1 2 3 4] 2)
     => 3
(elt '(1 2 3 4) 2)
     => 3
(char-to-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 duplicates aref (see section Functions that Operate on Arrays) and nth (see section Accessing Elements of Lists), except that it works for any kind of sequence.

Arrays

An array object refers directly to 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.

When you create an array, you must specify how many elements it has. The amount of space allocated depends on the number of elements. Therefore, it is impossible to change the size of an array once it is created. You cannot add or remove elements. However, you can replace an element with a different value.

Emacs defines two types of array, both of which are one-dimensional: strings and vectors. 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.

Both kinds of arrays share these characteristics:

In principle, if you wish to have an array of characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:

Functions that Operate on Arrays

In this section, we describe the functions that accept both strings and vectors.

Function: arrayp object

This function returns t if object is an array (i.e., either a vector or a string).

(arrayp [a])
=> t
(arrayp "asdf")
=> t

Function: aref array index

This function returns the indexth element of array. The first element is at index zero.

(setq primes [2 3 5 7 11 13])
     => [2 3 5 7 11 13]
(aref primes 4)
     => 11
(elt primes 4)
     => 11

(aref "abcdefg" 1)
     => 98           ; `b' is ASCII code 98.

See also the function elt, in section Sequences.

Function: aset array index object

This function sets the indexth element of array to be object. It returns object.

(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.

Function: fillarray array object

This function fills the array array with pointers to object, replacing any previous values. It returns array.

(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.

Vectors

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; its elements can be any Lisp objects. (The other kind of array provided in Emacs Lisp is the string, whose elements must be characters.) The main uses of vectors in Emacs are as syntax tables (vectors of integers) and 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.

The indices of the elements of a vector are numbered starting with zero in Emacs Lisp.

Vectors are printed with square brackets surrounding the elements in their order. Thus, a vector containing the symbols a, b and c is printed as [a b c]. 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. The elements of the vector are not evaluated. See section Self-Evaluating Forms.

Here are examples of 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:

Function: vectorp object

This function returns t if object is a vector.

(vectorp [a])
     => t
(vectorp "asdf")
     => nil

Function: vector &rest objects

This function creates and returns a vector whose elements are the arguments, objects.

(vector 'foo 23 [bar baz] "rats")
     => [foo 23 [bar baz] "rats"]
(vector)
     => []

Function: make-vector integer object

This function returns a new vector consisting of integer elements, each initialized to object.

(setq sleepy (make-vector 9 'Z))
     => [Z Z Z Z Z Z Z Z Z]

Function: vconcat &rest sequences

This function returns a new vector containing all the elements of the sequences. The arguments sequences may be lists, vectors, or strings. If no sequences are given, an empty vector is returned.

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)]

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 in this way, use format (see section Formatting Strings) or int-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 may be used 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])

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