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D.2.5 Arrays in Elisp

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. 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.

Emacs defines four types of array, all one-dimensional:

• “strings” (*note String Type::)
• “vectors” (*note Vector Type::)
• “bool-vectors” (*note Bool-Vector Type::)
• “char-tables” (*note Char-Table Type::)

Vectors and char-tables can hold elements of any type, but strings can only hold characters, and bool-vectors can only hold ‘t’ and ‘nil’.

Shared Characteristics

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.
• For purposes of evaluation, the array is a constant—i.e., it evaluates to itself.
• The elements of an array may be referenced or changed with the functions ‘aref’ and ‘aset’, respectively (*note Array Functions::).

Text Characters

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.

• Functions that Operate on Arrays