swift中高阶函数map、flatMap、filter、reduce

swift中高阶函数map、flatMap、filter、reduce

　　Swift相比于Objective-C又一个重要的优点，它对函数式编程提供了很好的支持，Swift提供了map、filter、reduce这三个高阶函数作为对容器的支持。1 map：可以对数组中的每一个元素做一次处理

// 计算字符串的长度
let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]
func stringCount(string: String) -> Int {
return string.characters.count
}
stringArray.map(stringCount)

stringArray.map({string -> Int in
return string.characters.count
})

// $0代表数组中的每一个元素
stringArray.map{
return $0.characters.count
}

2 flatMap与map不同之处：　　（1）flatMap返回后的数组中不存在nil，同时它会把Optional解包

let array = ["Apple", "Orange", "Puple", ""]

let arr1 = array.map { a -> Int? in
let length = a.characters.count
guard length > 0 else { return nil }
return length
}
arr1 // [{some 5}, {some 6}, {some 5}, nil]

let arr2 = array.flatMap { a-> Int? in
let length = a.characters.count
guard length > 0 else { return nil}
return length
}
arr2 // [5, 6, 5]

　　（2）flatMap还能把数组中存有数组的数组（二维数组、N维数组）一同打开变成一个新的数组

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr1 = array.map{ $0 }
arr1 // [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr2 = array.flatMap{ $0 }
arr2 // [1, 2, 3, 4, 5, 6, 7, 8, 9]

　　（3）flatMap也能把两个不同的数组合并成一个数组，这个合并的数组元素个数是前面两个数组元素个数的乘积

let fruits = ["Apple", "Orange", "Puple"]
let counts = [2, 3, 5]

let array = counts.flatMap { count in
fruits.map ({ fruit in
return fruit + "  \(count)"
})
}
array // ["Apple 2", "Orange 2", "Puple 2", "Apple 3", "Orange 3", "Puple 3", "Apple 5", "Orange 5", "Puple 5"]

3 filer：过滤，可以对数组中的元素按照某种规则进行一次过滤

// 筛选出字符串的长度小于10的字符串
let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]
func stringCountLess10(string: String) -> Bool {
return string.characters.count < 10
}
stringArray.filter(stringCountLess10)

stringArray.filter({string -> Bool in
return string.characters.count < 10
})
// $0表示数组中的每一个元素
stringArray.filter{
return $0.characters.count < 10
}

4 reduce：计算，可以对数组的元素进行计算

// 将数组中的每个字符串用‘、’拼接
let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]

func appendString(string1: String, string2: String) -> String {
return string1 == "" ? string2 : string1 + "、" + string2
}
// reduce方法中的第一个参数是初始值
stringArray.reduce("", appendString)

stringArray.reduce("", {(string1, string2) -> String in
return string1 == "" ? string2 : string1 + "、" + string2
})

// $0表示计算后的结果, $1表示数组中的每一个元素
stringArray.reduce("", {
return $0 == "" ? $1 : $0 + "、" + $1
})