6.2 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. See section 2.3.8 String Type, and 2.3.9 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 (see section 6.3 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:

• 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. See section 32.19 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. See section 4. 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. See section 21.7.1 Key Sequence Input.