Mapping in

About Mapping

Mapping refers to the application of a function to each element of a sequence to produce a new sequence.

mapcar can be used to map a function across a list of objects

(mapcar #'double '(1 2 3 4)) ; => (2 4 6 8)

Notice that the first argument is a function to call. The #' is a special quoting syntax to specify a function. (One can use 'double here but the use of #' is idiomatic and arguably "more correct".)

More than one list

Mapping is not limited to applying a function to a single sequence but can be done with multiple sequences. If N sequences are provided then the function must take N arguments.

(mapcar #'list '(1 2) '(a b) '("foo" "bar")) ; => ((1 a "foo") (2 b "bar"))

Other map functions

In addition to mapcar there are other functions for mapping across lists. mapcar is definitely the most common but it is good to know about the others.

maplist operates similarly to mapcar but operates on successive sublists rather than elements. First to the entire list, then to cdr, then to the cdr of that etc.:

(maplist #'identity '(a b c)) ; => ((A B C) (B C) (C))

mapc and mapl are like mapcar and maplist respectively but they do not collect the results and instead return the first list. This is used for side-effecting operations.

mapcan and mapcon are like mapcar and maplist but the result is flattened by use of nconc which is like append but destructive to all but its last argument.

Mapping on other sequences

Mapping is not limited to lists. Any sequence (covered in another concept) can be mapped over. In the case of non-list sequences the map function is used.

map takes 3 or more arguments:

• the first is the type of resulting sequence
• the second is the function to apply
• the third and remaining are the sequences to operate on.

(map 'list #'double '(1 2 3))        ; => (2 4 6)
(map 'string #'char-upcase "foobar") ; => "FOOBAR"

As a special case if nil is provided as the resulting sequence type then nil is returned. This (like mapc and mapl above) are used for side-effects.