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12.6 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, and mapconcat, 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: 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 (function (lambda (x) (format "%c" (1+ x))))
                     "HAL-8000"
                     "")
               ⇒ "IBM.9111"