# Swift中的高阶函数：sorted, map, reduce, forEach, flatMap, filter

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• 接受一个或多个函数作为输入
• 输出一个函数

### 1 sorted

``````public func sorted(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> [Element]
``````
``````let numbers = [1, 4, 2, 3]
let res = numbers.sorted {
\$0 < \$1
}
print(res.description)
``````

### 2 map

``````public func map<T>(_ transform: (Element) throws -> T) rethrows -> [T]
``````
``````let arr = [1, 2, 4]
let brr = arr.map {
"No." + String(\$0)
}
// brr = ["No.1", "No.2", "No.4"]
``````

### 3 reduce

Returns the result of combining the elements of the sequence using the given closure.（ 返回使用给定闭包组合序列元素的结果。）

``````public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result
``````
``````let crr = arr.reduce(0) {
(prevSum: Int, element: Int) in
return prevSum + element
}   // crr = 7
let drr = arr.reduce("") {
if \$0 == "" {
return String(\$1)
} else {
return \$0 + " " + String(\$1)
}
}
print(drr)      // drr = "1 2 4"
let res2 = [3,5,8,9].reduce(2) {
(prevSum: Int, element: Int) in
return prevSum + element
}
print(res2) // 27。 2+3+5+8+9 = 27
``````

### 4 forEach

Calls the given closure on each element in the sequence in the same order as a for-in loop.(与`for-in`类似)

``````public func forEach(_ body: (Element) throws -> Void) rethrows
``````
``````["Swift","OC","iOS","macOS"].forEach {
print(\$0)
}
``````

### 5 flatMap

Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.（返回一个数组，其中包含使用此序列的每个元素调用给定转换的连接结果。）

``````public func flatMap(_ transform: (Element) throws -> String?) rethrows -> [String]
``````
``````let collections = [[5, 2, 7], [4, 8], [9, 1, 3]]
let flat = collections.flatMap { \$0 }
// [5, 2, 7, 4, 8, 9, 1, 3]
``````

flatMap和map的区别是，对二维数组时flatMap有个降维处理，对于一位数组，两者没有明显区别

``````let numbersCompound = [[1,2,3],[4,5,6]]
let mapped = numbersCompound.map { \$0.map{ "\(\$0)-map" } }
print(mapped)       // [["1-map", "2-map", "3-map"], ["4-map", "5-map", "6-map"]]
let flatMapped = numbersCompound.flatMap { \$0.map{ "\(\$0)-flatMap" } }
print(flatMapped)   // ["1-flatMap", "2-flatMap", "3-flatMap", "4-flatMap", "5-flatMap", "6-flatMap"]

let numbers2 = [1, 2, 3, 4]
let mapres = numbers2.map { Array(repeating: \$0, count: \$0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]
let flatMapres = numbers2.flatMap { Array(repeating: \$0, count: \$0) }
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
``````

### 6 filter

Returns an array containing, in order, the elements of the sequence that satisfy the given predicate.（返回一个数组，该数组按顺序包含满足给定闭包的序列元素。）

``````public func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element]
``````
``````let err = arr.filter {
\$0 % 2 == 0
}
// err = [2, 4]
``````

playground文件在andyRon/LearnSwift