map, filter, reduce, flatMap, compactMap

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var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
var arr2 = arr.map { $0 * 2 }

// [2, 4]
var arr3 = arr.filter { $0 % 2 == 0 }

// 10
var arr4 = arr.reduce(0) { $0 + $1 }
// 10
var arr5 = arr.reduce(0, +)

func double(_ i: Int) -> Int { i * 2 }
var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
print(arr.map(double))

var arr = [1, 2, 3]
// [[1], [2, 2], [3, 3, 3]]
var arr2 = arr.map { Array.init(repeating: $0, count: $0) }

// [1, 2, 2, 3, 3, 3]
var arr3 = arr.flatMap { Array.init(repeating: $0, count: $0) }

var arr = ["123", "test", "jack", "-30"]
// [Optional(123), nil, nil, Optional(-30)]
var arr2 = arr.map { Int($0) }
// [123, -30]
var arr3 = arr.compactMap { Int($0) }

// 使用reduce实现map、filter的功能
var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
print(arr.map { $0 * 2 })
print(arr.reduce([]) { $0 + [$1 * 2] })

// [2, 4]
print(arr.filter { $0 % 2 == 0 })
print(arr.reduce([]) { $1 % 2 == 0 ? $0 + [$1] : $0 })

lazy的优化

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let arr = [1, 2, 3]
let result = arr.lazy.map {
    (i: Int) -> Int in
    print("mapping \(i)")
    return i * 2 
}
print("begin-----")
print("mapped", result[0])
print("mapped", result[1])
print("mapped", result[2])
print("end----")

begin-----
mapping 1
mapped 2
mapping 2
mapped 4
mapping 3
mapped 6
end----

Optional的map和flatMap

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var num1: Int? = 10
// Optional(20)
var num2 = num1.map { $0 * 2 }
var num3: Int? = nil
// nil
var num4 = num3.map { $0 * 2 }

var num1: Int? = 10
// Optional(Optional(20))
var num2 = num1.map { Optional.some($0 * 2) }
// Optional(20)
var num3 = num1.flatMap { Optional.some($0 * 2) }

var num1: Int? = 10
// num2、num3是等价的
var num2 = (num1 != nil) ? (num1! + 10) : nil
var num3 = num1.map { $0 + 10 }

var fmt = DateFormatter()
fmt.dateFormat = "yyyy-MM-dd"
var str: String? = "2011-09-10"
// old
var date1 = str != nil ? fmt.date(from: str!) : nil
// new
var date2 = str.flatMap(fmt.date)

var score: Int? = 98
// old
var str1 = score != nil ? "socre is \(score!)" : "No score"
// new
var str2 = score.map { "score is \($0)" } ?? "No score"

示例1

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struct Person {
    var name: String
    var age: Int
}

var items = [
    Person(name: "jack", age: 20),
    Person(name: "rose", age: 21),
    Person(name: "kate", age: 22) 
]
// old
func getPerson1(_ name: String) -> Person? {
    let index = items.firstIndex { $0.name == name }
    return index != nil ? items[index!] : nil
}

// new
func getPerson2(_ name: String) -> Person? {
    return items.firstIndex { $0.name == name }.map { items[$0] }
}

示例2

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struct Person {
    var name: String
    var age: Int
    init?(_ json: [String : Any]) {
        guard let name = json["name"] as? String,
        let age = json["age"] as? Int else {
            return nil
        }
    self.name = name
    self.age = age
    } 
}

var json: Dictionary? = ["name" : "Jack", "age" : 10]
// old
var p1 = json != nil ? Person(json!) : nil

// new
var p2 = json.flatMap(Person.init)

参考

  • 李明杰老师课件