Swift Functional Programming - Second Edition

The filter function

The filter function takes a function that, given an element in array, returns Bool, indicating whether the element should be included in the resulting array. The filter method is declared as follows in Swift standard library:

public func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element] 

The definition is similar to the map method with the following differences:

• The filter function takes a closure that receives elements of itself and returns a Bool value
• The result of the filter method will be an array of its own type

Let's examine the following code to understand how it works:

let numbers = [10, 30, 91, 50, 100, 39, 74] let evenNumbers = numbers.filter { $0 % 2 == 0 } 

The resulting evenNumbers ...