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Lisp introduced maplist as a first mapping function around 1959 |
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==Language comparison==
The map function originated in [[functional programming]] languages.
The map function originated in [[functional programming]] languages but is today supported (or may be defined) in many [[procedural programming|procedural]], [[object oriented]], and [[multi-paradigm]] languages as well: In [[C++]]'s [[Standard Template Library]], it is called <code>std::transform</code>, in C# (3.0)'s LINQ library, it is provided as an extension method called <code>Select</code>. Map is also a frequently used operation in high level languages such as [[CFML]], [[Perl]], [[Python (programming language)|Python]] and [[Ruby (programming language)|Ruby]]; the operation is called <code>map</code> in all four of these languages. A <code>collect</code> alias for <code>map</code> is also provided in Ruby (from [[Smalltalk]]). [[Common Lisp]] provides a family of map-like functions; the one corresponding to the behavior described here is called <code>mapcar</code> (<code>-car</code> indicating access using the [[CAR and CDR|CAR operation]]). There are also languages with syntactic constructs providing the same functionality as the map function.▼
The Lisp programming language introduced a map function called <code>maplist</code><ref>[http://www.softwarepreservation.org/projects/LISP/MIT/LISP_Prog_Man-Mar_1959.pdf/view J. McCarthy, K. Maling, S. Russell, N. Rochester, S. Goldberg, J. Slagle. LISP Programmer's Manual. March-April, 1959]</ref> in 1959, with slightly different versions already appearing in 1958<ref>[http://www.softwarepreservation.org/projects/LISP/MIT/AIM-004.pdf/view J. McCarthy: Symbol Manipulating Language - Revisions of the Language. AI Memo No. 4, October 1958]</ref>. This is the original definition for <code>maplist</code>, mapping a function of successive rest lists..
<source>
maplist[x;f] = [null[x] -> NIL;T -> cons[f[x];maplist[cdr[x];f]]]
</source>
The function <code>maplist</code> is still available in newer Lisps like [[Common Lisp]]<ref>[http://www.lispworks.com/documentation/HyperSpec/Body/f_mapc_.htm Function MAPC, MAPCAR, MAPCAN, MAPL, MAPLIST, MAPCON in ANSI Common Lisp]</ref>, though functions like <code>mapcar</code> or the more generic <code>map</code> would be preferred.
Squaring the elements of a list using <code>maplist</code> would be written in s-expression notation like this:
<source lang="lisp">
(maplist (function (lambda (l)
(sqr (car l))))
'(1 2 3 4 5))
</source>
▲
Map is sometimes generalized to accept dyadic (2-argument) functions that can apply a user-supplied function to corresponding elements from two lists; some languages use special names for this, such as ''map2'' or ''zipWith''. Languages using explicit [[variadic function]]s may have versions of map with variable [[arity]] to support ''variable-arity'' functions. Map with 2 or more lists encounters the issue of handling when the lists are of different lengths. Various languages differ on this; some raise an exception, some stop after the length of the shortest list and ignore extra items on the other lists; some continue on to the length of the longest list, and for the lists that have already ended, pass some placeholder value to the function indicating no value.
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