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map/filter/reduce
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
// 可简写为
var arr5 = arr.reduce(0, +)
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map接收函数作为参数
func double(_ i: Int) -> Int { i * 2 }
var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
print(arr.map(double))
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compactMap默认解包并去除nil
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函数的功能
var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
print(arr.map { $0 * 2 })
// 等价于👆函数功能
print(arr.reduce([]) { $0 + [$1 * 2] })
// [2, 4]
print(arr.filter { $0 % 2 == 0 })
// 等价于👆函数功能
print(arr.reduce([]) { $1 % 2 == 0 ? $0 + [$1] : $0 })
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map,flatMap功能对比
var arr = [1, 2, 3]
// [[1], [2, 2], [3, 3, 3]] 拿到什么(数组)，返回数组(二维数组)就放什么，不做处理。
var arr2 = arr.map { Array.init(repeating: $0, count: $0) }
// [1, 2, 2, 3, 3, 3] 将拿到的数组中的元素取出放到返回数组(一维数组)中。扁平化处理。
var arr3 = arr.flatMap { Array.init(repeating: $0, count: $0) }
- 用
lazy的优化map
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----")
// 第一次用到时，才会执行相应逻辑
begin-----
mapping 1
mapped 2
mapping 2
mapped 4
mapping 3
mapped 6
end----
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map和flatMap可选值Optional处理中的应用
var num1: Int? = 10
// Optional(20)
var num2 = num1.map { $0 * 2 }
var num3: Int? = nil
// nil，发现$0为nil直接返回nil
var num4 = num3.map { $0 * 2 }
map 和 flatMap对比
var num1: Int? = 10
// Optional(Optional(20)), map还是会对可选值再包装一层
var num2 = num1.map { Optional.some($0 * 2) }
// Optional(20), flatMap发现已经是可选值，就不会再包装了
var num3 = num1.flatMap { Optional.some($0 * 2) }
可用于简化代码
// 示例一：
var num1: Int? = 10
var num2 = (num1 != nil) ? (num1! + 10) : nil
var num3 = num1.map { $0 + 10 }
// num2、num3是等价的
// 示例二：
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)
// 示例三：
var score: Int? = 98
// old
var str1 = score != nil ? "socre is \(score!)" : "No score"
// new
var str2 = score.map { "score is \($0)" } ?? "No score"
// 示例四：
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] }
}
// 示例五：
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)
