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 true.
Example In Haskell, the code example
filter even [1..10]
evaluates to the list 2, 4, …, 10 by applying the predicate even 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 filter (not . even) [1..10] evaluates to the list 1, 3, …, 9 by collecting those elements of the list of integers 1, 2, …, 10 for which the predicate even 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 X = [0, 5, 8, 3, 2, 1] according to the function:
This function express that if 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,filter in the Haskell Standard Prelude OCaml,filter in the OCaml standard library module list Standard ML, or Erlang.filter/2 in the Erlang STDLIB Reference Manual documentation of the module lists Common Lisp provides the functions remove-if and remove-if-not.Function REMOVE, REMOVE-IF, REMOVE-IF-NOT, DELETE, DELETE-IF, DELETE-IF-NOT in the Common Lisp HyperSpec Scheme Requests for Implementation (SRFI) 1 provides an implementation of filter for the language Scheme.filter in SRFI 1 C++ provides the algorithms remove_if (mutating) and remove_copy_if (non-mutating); C++11 additionally provides copy_if (non-mutating).remove_if and remove_copy_if in the SGI Standard Template Library (STL) spec Smalltalk provides the select: method for collections. Filter can also be realized using list comprehensions in languages that support them.
In Haskell, filter can be implemented like this: filter :: (a -> Bool) -> [a] -> [a] filter _ [] = [] filter p (x:xs) = [x | p x] ++ filter p xs
Here, [] denotes the empty list, ++ the list concatenation operation, and [x | p x] denotes a list conditionally holding a value, x, if the condition p x holds (evaluates to True).
+ Filter in various languages Language Filter Notes APL (pred array)/arrayorpredarray The second example is an APL dop. C# 3.0 ienum.Where(pred)orThe where clause Where is an extension method ienum is an IEnumerable Similarly in all .NET languages CFML obj.filter(func) Where obj is an array or a structure. The func receives as an argument each element's value. Clojure (filter predicate list)clojure.core/filter on ClojureDocs Or, via list comprehension: (for [x list :when (pred x)] x) Common Lisp (remove-if inverted-pred list)(remove-if (complement pred) list)(remove-if-not pred list) The function remove-if-not has been deprecated in favor of the equivalent remove-if where the predicate is complemented.Function COMPLEMENT in the Common Lisp HyperSpec Thus the filter should be written or more simply: where evenp returns the inverted value of oddp.Function EVENP, ODDP in the Common Lisp HyperSpec C++ std::remove_copy_if(begin, end, result, prednot)std::copy_if(begin, end, result, pred) (C++11) in header <algorithm> begin, end, result are iterators predicate is reversed D std.algorithm.filter!(pred)(list) Erlang lists:filter(Fun, List) Or, via list comprehension: Groovy list.findAll(pred) Haskell filter pred list Or, via list comprehension: [x | x <- list, pred x] Haxe list.filter(pred)Lambda.filter(list, pred) Or, via list comprehension: [x | x <- list, pred x] J (#~ pred) list An example of a monadic hook. # is copy, ~ reverses arguments. (f g) y = y f (g y) Julia filter(pred, array) The filter function also accepts dict datatype. Or, via list comprehension: [x for x in array if pred(x)] Java 8+ stream.filter(pred) JavaScript 1.6 array.filter(pred) Kotlin array.filter(pred) Mathematica Select[list, pred] Objective-C (Cocoa in Mac OS X 10.4+) [array filteredArrayUsingPredicate:pred] pred is an NSPredicate object, which may be limited in expressiveness F#, OCaml, Standard ML List.filter pred list PARI/GP select(expr, list) The order of arguments is reversed in v. 2.4.2. Perl grep block list grep expr, list PHP array_filter(array, pred) Prolog Since ISO/IEC 13211-1:1995/Cor.2:2012ISO/IEC 13211-1:1995/Cor 2:2012 the core standard contains closure application via call/N Python filter(func, list) Or, via list comprehension: list if pred(x)]. In Python 3, filter was changed to return an iterator rather than a list. The complementary functionality, returning an iterator over elements for which the predicate is false, is also available in the standard library as filterfalse in the itertools module. Ruby enum.find_all {block} enum.select {block} enum is an Enumeration Rust iterator.filter(pred) iterator is an Iterator and the filter method returns a new iterator; pred is a function (specifically FnMut) that receives the iterator's item and returns a bool S, R Filter(pred,array)array[pred(array)] In the second case, pred must be a vectorized function Scala list.filter(pred) Or, via for-comprehension: for(x <- list; if pred) yield x Scheme R6RS (filter pred list)(remove inverted pred list)(partition pred list list) Smalltalk aCollection select: aBlock Swift array.filter(pred) filter(sequence, pred) XPath, XQuery list[block] filter(list, func) In block the context item . holds the current value
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 dropWhileHaskell filter dropWhile and partitionHaskell filter partition) 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).
See also
Map (higher-order function) List comprehension Guard (computing)
References