Swift_Array的几个高级函数map, filter, reduce

map映射一个新数组

在这个例子中,“map”将第一个数组中的 element 转换为小写的字符串,然后计算他们的characters。

let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let lowercaseNames = cast.map { $0.lowercased() }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.characters.count }
// 'letterCounts' == [6, 6, 3, 4]

API

    /// - Parameter transform: A mapping closure. `transform` accepts an    ///   element of this sequence as its parameter and returns a transformed    ///   value of the same or of a different type.    /// - Returns: An array containing the transformed elements of this    ///   sequence.    public func map<T>(_ transform: (Element) throws -> T) rethrows -> [T]

filter 提取数组中满足条件的元素
在这个例子中,“filter”用于只包括短于5个字符的名称。

let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let shortNames = cast.filter { $0.characters.count < 5 }
print(shortNames)
// Prints "["Kim", "Karl"]"

API

/// - Parameter shouldInclude: A closure that takes an element of the
///   sequence as its argument and returns a Boolean value indicating
///   whether the element should be included in the returned array.
/// - Returns: An array of the elements that `includeElement` allowed.
public func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element]

reduce 将数组元素组合计算为一个值。这个例子展示了如何寻找一组数字的总和。

let numbers = [1, 2, 3, 4]
let addTwo: (Int, Int) -> Int = { x, y in x + y }
let numberSum = numbers.reduce(0, addTwo)
// 'numberSum' == 10

let sum =  numbers.reduce(0) { (sum, x) -> Int in
sum + x
}
// 'sum' == 10

//可以写成下面形式sum = numbers.reduce(0,{$0 + $1})sum = numbers.reduce(0,+)

API

/// - Parameters:
///   - initialResult: the initial accumulating value.
///   - nextPartialResult: A closure that combines an accumulating
///     value and an element of the sequence into a new accumulating
///     value, to be used in the next call of the
///     `nextPartialResult` closure or returned to the caller.
/// - Returns: The final accumulated value.
public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result