In functional programming, filter is a higher-order function that processes a data structure (usually a list) in some order to produce a new data structure containing exactly those elements of the original data structure for which a given predicate returns the Boolean value.

Example

In Haskell, the code example evaluates to the list 2, 4, …, 10 by applying the predicate to every element of the list of integers 1, 2, …, 10 in that order and creating a new list of those elements for which the predicate returns the Boolean value true, thereby giving a list containing only the even members of that list. Conversely, the code example evaluates to the list 1, 3, …, 9 by collecting those elements of the list of integers 1, 2, …, 10 for which the predicate returns the Boolean value false (with being the function composition operator).

Visual example

Below, you can see a view of each step of the filter process for a list of integers according to the function : This function express that if x is even the return value is, otherwise it's. This is the predicate.

Language comparison

Filter is a standard function for many programming languages, e.g., Haskell, OCaml, Standard ML, or Erlang. Common Lisp provides the functions and. Scheme Requests for Implementation (SRFI) 1 provides an implementation of filter for the language Scheme. C++ provides the algorithms (mutating) and (non-mutating); C++11 additionally provides (non-mutating). Smalltalk provides the method for collections. Filter can also be realized using list comprehensions in languages that support them. In Haskell, can be implemented like this: Here, denotes the empty list, the list concatenation operation, and denotes a list conditionally holding a value, , if the condition holds (evaluates to ).

Variants

Filter creates its result without modifying the original list. Many programming languages also provide variants that destructively modify the list argument instead for faster performance. Other variants of filter (e.g., Haskell and ) are also common. A common memory optimization for purely functional programming languages is to have the input list and filtered result share the longest common tail (tail-sharing).