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max方法利用max方法，可直接遍历出数组中最大值和最小值 var arr = [1, 2, 3, 4] //// 返回数组中的最大值 和 最小值 //@inlinable public func max(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> Element? // 根据返回值true和false返回数组中的最大值和最小值,是一个可选值 var maxElement = arr.max { (temp1, temp2) -> Bool in return temp1 < temp2 } map方法map方法可遍历整个数组，并对数组个每个元素进行操作，返回处理后的数组 /// Returns an array containing the results of mapping the given closure /// over the sequence's elements. /// /// In this example, `map` is used first to convert the names in the array /// to lowercase strings and then to count their characters. /// /// let cast = ["Vivien", "Marlon", "Kim", "Karl"] /// let lowercaseNames = cast.map { $0.lowercased() } /// // 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"] /// let letterCounts = cast.map { $0.count } /// // 'letterCounts' == [6, 6, 3, 4] /// /// - 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. //@inlinable public func map<T>(_ transform: (Element) throws -> T) rethrows -> [T] var arr3 = arr.map { (temp) -> Int in return temp * 2 } print(arr3) //[2, 4, 6, 8] filter方法filter方法可遍历整个数组，并依据设置的条件，对数组筛选并返回新的数组 // 根据条件进行过滤 var arr4 = arr.filter { (temp) -> Bool in return temp % 2 == 0 } print(arr4) // [2, 4] reduce方法reduce设置初始参数，根据传入的闭包操作，返回结果。例如相乘或者相加 /// Returns the result of combining the elements of the sequence using the /// given closure. // @inlinable public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result var arr5 = arr.reduce(5) { (result, temp) -> Int in // 5 * 1 * 2 * 3 * 4 return result * temp } //简便写法 var arr6 = arr.reduce(5, *) print(arr5) // 120 map和flatMap区别// @inlinable public func map<T>(_ transform: (Element) throws -> T) rethrows -> [T] var arr7 = arr.map { Array.init(repeating: $0, count: $0) } print(arr7) /// Returns an array containing the concatenated results of calling the /// given transformation with each element of this sequence. /// In fact, `s.flatMap(transform)` is equivalent to /// `Array(s.map(transform).joined())`. //@inlinable public func flatMap<SegmentOfResult>(_ transform: (Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence var arr8 = arr.flatMap { Array.init(repeating: $0, count: $0) } print(arr8) //[[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]] //[1, 2, 2, 3, 3, 3, 4, 4, 4, 4] 两者相比较，正如文档注释一下flatMap会拼接成一个数组 map和compactMap区别var stringArr = ["123", "test", "haha", "-234"] var arr2 = stringArr.map { Int($0) } print(arr2) /// Returns an array containing the non-`nil` results of calling the given /// transformation with each element of this sequence. var arr3 = stringArr.compactMap { Int($0) } print(arr3) //[Optional(123), nil, nil, Optional(-234)] //[123, -234] map会完成所传入的闭包操作，若是失败会赋值为空，成功会返回一个可选值 compactMap根据闭包的操作，若是失败则被移除，返回的解包后的值 Array中lazy方法let result = arr.map { (temp) -> Int in
print("map \(temp)")
return temp * 2
}
print("begin--")
print("test", result[0])
print("test", result[1])
print("test", result[2])
print("end")
数组的遍历未使用lazy时候，打印结果如下: map 1 map 2 map 3 map 4 begin-- test 2 test 4 test 6 end 会先按照顺序执行，执行完map遍历，再执行下面的打印 而对于利用lazy遍历来说，代码如下： let result = arr.lazy.map { (temp) -> Int in
print("map \(temp)")
return temp * 2
}
print("begin--")
print("test", result[0])
print("test", result[1])
print("test", result[2])
print("end")
打印结果如下: begin-- map 1 test 2 map 2 test 4 map 3 test 6 end 会发现数组里面的打印会在该数组的元素被使用的时候才会调用，这个方法适用于当数组元素的数量比较大时候 |
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