GNU Info

(elisp)Arrays

---

Next: Array Functions Prev: Sequence Functions Up: Sequences Arrays Vectors

---

Enter node , (file) or (file)node

Arrays
======

   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.  Each type of array has its own
read syntax.  Note: String Type, and Note: Vector Type.

   All four kinds of array share these characteristics:

   * The first element of an array has index zero, the second element
     has index 1, and so on.  This is called "zero-origin" indexing.
     For example, an array of four elements has indices 0, 1, 2, and 3.

   * The length of the array is fixed once you create it; you cannot
     change the length of an existing array.

   * The array is a constant, for evaluation--in other words, it
     evaluates to itself.

   * The elements of an array may be referenced or changed with the
     functions `aref' and `aset', respectively (Note: Array
     Functions).

   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:

   * They occupy one-fourth the space of a vector of the same elements.

   * Strings are printed in a way that shows the contents more clearly
     as text.

   * Strings can hold text properties.  Note: Text Properties.

   * Many of the specialized editing and I/O facilities of Emacs accept
     only strings.  For example, you cannot insert a vector of
     characters into a buffer the way you can insert a string.  Note:
     Strings and Characters.

   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.  Note: Key
Sequence Input.