13.6 Mapping Functions

A mapping function applies a given function (not a special form or macro) to each element of a list or other collection. Emacs Lisp has several such functions; this section describes mapcar, mapc, mapconcat, and mapcan, which map over a list. See Definition of mapatoms, for the function mapatoms which maps over the symbols in an obarray. See Definition of maphash, for the function maphash which maps over key/value associations in a hash table.

These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function map-char-table (see Char-Tables).

function mapcar function sequence

mapcar applies function to each element of sequence in turn, and returns a list of the results.

The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence. For example:

(mapcar 'car '((a b) (c d) (e f)))
     ⇒ (a c e)
(mapcar '1+ [1 2 3])
     ⇒ (2 3 4)
(mapcar 'string "abc")
     ⇒ ("a" "b" "c")

;; Call each function in my-hooks.
(mapcar 'funcall my-hooks)

(defun mapcar* (function &rest args)
  "Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
  ;; If no list is exhausted,
  (if (not (memq nil args))
      ;; apply function to CARs.
      (cons (apply function (mapcar 'car args))
            (apply 'mapcar* function
                   ;; Recurse for rest of elements.
                   (mapcar 'cdr args)))))

(mapcar* 'cons '(a b c) '(1 2 3 4))
     ⇒ ((a . 1) (b . 2) (c . 3))

function mapcan function sequence

This function applies function to each element of sequence, like mapcar, but instead of collecting the results into a list, it returns a single list with all the elements of the results (which must be lists), by altering the results (using nconc; see Rearrangement). Like with mapcar, sequence can be of any type except a char-table.

;; Contrast this:
(mapcar 'list '(a b c d))
     ⇒ ((a) (b) (c) (d))
;; with this:
(mapcan 'list '(a b c d))
     ⇒ (a b c d)

function mapc function sequence

mapc is like mapcar except that function is used for side-effects only—the values it returns are ignored, not collected into a list. mapc always returns sequence.

function mapconcat function sequence separator

mapconcat applies function to each element of sequence; the results, which must be sequences of characters (strings, vectors, or lists), are concatenated into a single string return value. Between each pair of result sequences, mapconcat inserts the characters from separator, which also must be a string, or a vector or list of characters. See Sequences Arrays Vectors.

The argument function must be a function that can take one argument and returns a sequence of characters: a string, a vector, or a list. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string.

(mapconcat 'symbol-name
           '(The cat in the hat)
           " ")
     ⇒ "The cat in the hat"

(mapconcat (lambda (x) (format "%c" (1+ x)))
           "HAL-8000"
           "")
     ⇒ "IBM.9111"