Info Catalog elisp: Sequence Functions elisp: Sequences Arrays Vectors elisp: Array Functions

elisp: Arrays

 
 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, the time to access an element
 of a list is proportional to the position of that element in the list.
 
    Emacs defines four types of array, all one-dimensional: “strings”
 (String Type), “vectors” (Vector Type), “bool-vectors”
 (Bool-Vector Type), and “char-tables” (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’.
 
    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 (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.  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.  
      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.  Key
 Sequence Input.
 

Info Catalog elisp: Sequence Functions elisp: Sequences Arrays Vectors elisp: Array Functions