Map (higher-order function): Difference between revisions

Revision as of 07:07, 25 February 2019

Revision as of 22:23, 25 February 2019

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Today mapping functions are supported (or may be defined) in many [[procedural programming|procedural]], [[Object-oriented programming|object-oriented]], and [[multi-paradigm]] languages as well: In [[C++]]'s [[Standard Template Library]], it is called <code>std::transform</code>, in [[C Sharp (programming language)|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 [[ColdFusion Markup Language]] (CFML), [[Perl]], [https://uiv.me/blog/2019/02/25/lua-map/ Lua], [[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.

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.