1.Array的常見操作
do {
let arr = [1, 2, 3, 4]
let arr2 = arr.map { $0 * 2 }
// [2, 4, 6, 8]
print(arr2)
// [2, 4]
let arr3 = arr.filter { $0 % 2 == 0 }
// 10
print(arr3)
let arr4 = arr.reduce(0) { $0 + $1 }
// 10
print(arr4)
let arr5 = arr.reduce(0, +)
print(arr5)
}
do {
func double(_ i: Int) -> Int { i * 2 }
let arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
print(arr.map(double))
}
do {
let arr = [1, 2, 3]
let arr2 = arr.map { Array.init(repeating: $0, count: $0) }
// [[1], [2, 2], [3, 3, 3]]
print(arr2)
let arr3 = arr.flatMap { Array.init(repeating: $0, count: $0) }
// [1, 2, 2, 3, 3, 3]
print(arr3)
}
do {
let arr = ["123", "test", "jack", "-30"]
let arr2 = arr.map { Int($0) }
// [Optional(123), nil, nil, Optional(-30)]
print(arr2)
let arr3 = arr.compactMap { Int($0) }
// [123, -30]
print(arr3)
}
do {
// 使用reduce實(shí)現(xiàn)map、filter的功能
let 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 })
}
2.lazy的優(yōu)化
do {
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----
*/
}
3.Optional的map和flatMap
do {
let num1: Int? = 10
let num2 = num1.map { $0 * 2 }
// Optional(20)
print(num2 as Any)
let num3: Int? = nil
let num4 = num3.flatMap { $0 * 2 }
// nil
print(num4 as Any)
}
do {
/// 字符串 轉(zhuǎn) 日期
let fmt = DateFormatter()
fmt.dateFormat = "yyyy-MM-dd"
let str: String? = "2011-09-10"
// old
let date1 = str != nil ? fmt.date(from: str!) : nil
print(date1 as Any)
// new
let date2 = str.flatMap(fmt.date)
print(date2 as Any)
}
do {
let num1: Int? = 10
let num2 = num1.map { Optional.some($0 * 2) }
// Optional(Optional(20))
print(num2 as Any)
let num3 = num1.flatMap { Optional.some($0 * 2) }
// Optional(20)
print(num3 as Any)
}
do {
let score: Int? = 98
// old
let str1 = score != nil ? "socre is \(score!)" : "No score"
print(str1)
// new
let str2 = score.map { "score is \($0)" } ?? "No score"
print(str2)
}
do {
let num1: Int? = 10
let num2 = (num1 != nil) ? (num1! + 10) : nil
let num3 = num1.map { $0 + 10 }
// num2嫩与、num3是等價(jià)的
print(num2 as Any)
print(num3 as Any)
}
do {
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] }
}
}
do {
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
}
}
let json: Dictionary? = ["name" : "Jack", "age" : 10] // old
let p1 = json != nil ? Person(json!) : nil
// new
let p2 = json.flatMap(Person.init)
print(p1 as Any)
print(p2 as Any)
}
4.函數(shù)式編程(Funtional Programming)
- 函數(shù)式編程(Funtional Programming，簡(jiǎn)稱FP)是一種編程范式，也就是如何編寫程序的方法論
- 主要思想:把計(jì)算過程盡量分解成一系列可復(fù)用函數(shù)的調(diào)用
- 主要特征:函數(shù)是“第一等公民”
- 函數(shù)與其他數(shù)據(jù)類型一樣的地位仙畦，可以賦值給其他變量追逮，也可以作為函數(shù)參數(shù)豺谈、函數(shù)返回值
- 函數(shù)式編程最早出現(xiàn)在LISP語(yǔ)言，絕大部分的現(xiàn)代編程語(yǔ)言也對(duì)函數(shù)式編程做了不同程度的支持绿语，比如
- Haskell、JavaScript候址、Python吕粹、Swift、Kotlin岗仑、Scala等
- 函數(shù)式編程中幾個(gè)常用的概念
- Higher-Order Function匹耕、Function Currying
- Functor、Applicative Functor荠雕、Monad
4.1.FP實(shí)踐 – 傳統(tǒng)寫法
// 假設(shè)要實(shí)現(xiàn)以下功能:[(num + 3) * 5 - 1] % 10 / 2
do {
let num = 1
func add(_ v1: Int, _ v2: Int) -> Int {
v1 + v2
}
func sub(_ v1: Int, _ v2: Int) -> Int {
v1 - v2
}
func multiple(_ v1: Int, _ v2: Int) -> Int {
v1 * v2
}
func divide(_ v1: Int, _ v2: Int) -> Int {
v1 / v2
}
func mod(_ v1: Int, _ v2: Int) -> Int {
v1 % v2
}
divide(mod(sub(multiple(add(num, 3), 5), 1), 10), 2)
}
infix operator >>> : AdditionPrecedence
func >>><A, B, C>(_ f1: @escaping (A) -> B,_ f2: @escaping (B) -> C) -> (A) -> C {
{ f2(f1($0)) }
}
// FP實(shí)踐 – 函數(shù)式寫法
do {
let num = 1
func add(_ v: Int) -> (Int) -> Int {
{ $0 + v }
}
func sub(_ v: Int) -> (Int) -> Int {
{ $0 - v }
}
func multiple(_ v: Int) -> (Int) -> Int {
{ $0 * v }
}
func divide(_ v: Int) -> (Int) -> Int {
{ $0 / v }
}
func mod(_ v: Int) -> (Int) -> Int {
{ $0 % v }
}
let fn = add(3) >>> multiple(5) >>> sub(1) >>> mod(10) >>> divide(2)
fn(num)
}
4.3高階函數(shù)(Higher-Order Function)
高階函數(shù)是至少滿足下列一個(gè)條件的函數(shù):
- 接受一個(gè)或多個(gè)函數(shù)作為輸入(map稳其、filter、reduce等)
- 返回一個(gè)函數(shù)
- FP中到處都是高階函數(shù)
do {
func add(_ v: Int) -> (Int) -> Int { { $0 + v } }
}
4.4柯里化(Currying)
- 什么是柯里化?
-
將一個(gè)接受多參數(shù)的函數(shù)變換為一系列只接受單個(gè)參數(shù)的函數(shù)
image.png - Array炸卑、Optional的map方法接收的參數(shù)就是一個(gè)柯里化函數(shù)
-
//柯里化
do {
func add1(_ v1: Int, _ v2: Int) -> Int { v1 + v2 }
add1(10, 20)
func add(_ v: Int) -> (Int) -> Int { { $0 + v } }
add(10)(20)
}
func add3(_ v1: Int, _ v2: Int) -> Int { v1 + v2 }
func add4(_ v1: Int, _ v2: Int, _ v3: Int) -> Int { v1 + v2 + v3 }
func currying<A, B, C>(_ fn: @escaping (A, B) -> C) -> (B) -> (A) -> C {
{ b in { a in fn(a, b) } }
}
func currying1<A, B, C, D>(_ fn: @escaping (A, B, C) -> D) -> (C) -> (B) -> (A) -> D {
{ c in
{ b in
{ a in
fn(a, b, c)
}
}
}
}
let curriedAdd1 = currying(add3)
print(curriedAdd1(10)(20))
let curriedAdd2 = currying1(add4)
print(curriedAdd2(10)(20)(30))
prefix func ~<A, B, C>(_ fn: @escaping (A, B) -> C) -> (B) -> (A) -> C {
{ b in { a in fn(a, b) } }
}
func add0(_ v1: Int, _ v2: Int) -> Int {
v1 + v2
}
func sub0(_ v1: Int, _ v2: Int) -> Int {
v1 - v2
}
func multiple0(_ v1: Int, _ v2: Int) -> Int {
v1 * v2
}
func divide0(_ v1: Int, _ v2: Int) -> Int {
v1 / v2
}
func mod0(_ v1: Int, _ v2: Int) -> Int {
v1 % v2
}
let num = 1
let fn0 = (~add0)(3) >>> (~multiple0)(5) >>> (~sub0)(1) >>> (~mod0)(10) >>> (~divide0)(2)
fn0(num)
