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GNU Emacs Lisp Reference Manual

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12.6 Mapping Functions

A mapping function applies a given function to each element of a list or other collection. Emacs Lisp has several such functions; mapcar and mapconcat, which scan a list, are described here. See section 8.3 Creating and Interning Symbols, for the function mapatoms which maps over the symbols in an obarray. See section 7.2 Hash Table Access, 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 section 6.6 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 'char-to-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 strings, are concatenated. Between each pair of result strings, mapconcat inserts the string separator. Usually separator contains a space or comma or other suitable punctuation.

The argument function must be a function that can take one argument and return a string. 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"


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