Swift中map、flatMap、compactMap、filer、reduce的使用总结

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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.count
}
stringArray.map(stringCount)
 
stringArray.map({string -> Int in
    return string.count
})
 
// $0代表数组中的每一个元素
stringArray.map{
    return $0.count
}

let arr = [1,2,3,4,5,6,7]
func addOne(n1: Int) ->Int{
    return n1+1;
}
let arr1 = arr.map(addOne)
print(arr1)//[2, 3, 4, 5, 6, 7, 8]

let arr = [1,2,3,4,5,6,7]
let arr1 = arr.map{(n1) in n1+1}
print(arr1) //[2, 3, 4, 5, 6, 7, 8]

let arr = [1,2,3,4,5,6,7]
let arr1 = arr.map{$0+1}
print(arr1) //[2, 3, 4, 5, 6, 7, 8]

2.flatMap与map的区别

(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.count
    guard length > 0 else { return nil }
    return length  
}
arr1 // [5, 6, 5, nil]
 
let arr2 = array.flatMap { a-> Int? in
    let length = a.count
    guard length > 0 else { return nil}
    return length    
}    
arr2 // [5, 6, 5]

（2）flatMap还能把多维数组变成一维数组：

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"]

4、compactMap: 当后面的闭包中返回Optional的时候，在Swift 4.1之后应该使用compactMap代替map。
5、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.count < 10
}
stringArray.filter(stringCountLess10)
 
stringArray.filter({string -> Bool in
    return string.count < 10
})
 
// $0表示数组中的每一个元素
stringArray.filter{
    return $0.count < 10
}

6、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
})