swift中高階函數(shù)map佩研、flatMap、filter霞揉、reduce
Swift相比于Objective-C又一個(gè)重要的優(yōu)點(diǎn)韧骗，它對(duì)函數(shù)式編程提供了很好的支持，Swift提供了map零聚、filter袍暴、reduce這三個(gè)高階函數(shù)作為對(duì)容器的支持。
1 map：可以對(duì)數(shù)組中的每一個(gè)元素做一次處理
// 計(jì)算字符串的長度
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
})
stringArray.map{
? ? return $0.count
}
2 flatMap與map不同之處：
×ブⅰ（1）flatMap返回后的數(shù)組中不存在nil政模，同時(shí)它會(huì)把Optional解包
let array0 = ["Apple", "Orange", "Puple", ""]
let arr01 = array0.map { a -> Int? in
? ? let length = a.count
? ? guard length > 0 else { return nil }
? ? return length
}
arr01 // [{some 5}, {some 6}, {some 5}, nil]
let arr02 = array0.compactMap { a-> Int? in
? ? let length = a.count
? ? guard length > 0 else { return nil}
? ? return length
}
arr02 // [5, 6, 5]
????（2）flatMap還能把數(shù)組中存有數(shù)組的數(shù)組（二維數(shù)組、N維數(shù)組）一同打開變成一個(gè)新的數(shù)組
let array1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let arr11 = array1.map{ $0 }
arr11 // [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let arr12 = array1.flatMap{ $0 }
arr12 // [1, 2, 3, 4, 5, 6, 7, 8, 9]
????（3）flatMap也能把兩個(gè)不同的數(shù)組合并成一個(gè)數(shù)組蚂会，這個(gè)合并的數(shù)組元素個(gè)數(shù)是前面兩個(gè)數(shù)組元素個(gè)數(shù)的乘積
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：過濾淋样，可以對(duì)數(shù)組中的元素按照某種規(guī)則進(jìn)行一次過濾
// 篩選出字符串的長度小于10的字符串
let strArray = ["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表示數(shù)組中的每一個(gè)元素
stringArray.filter{
? ? return $0.count < 10
}
4 reduce：計(jì)算，可以對(duì)數(shù)組的元素進(jìn)行計(jì)算
// 將數(shù)組中的每個(gè)字符串用‘胁住、’拼接
let strArray1 = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]
func appendString(string1: String, string2: String) -> String {
? ? return string1 == "" ? string2 : string1 + "趁猴、" + string2
}
// reduce方法中的第一個(gè)參數(shù)是初始值
strArray1.reduce("", appendString)
strArray1.reduce("", {(string1, string2) -> String in
? ? return string1 == "" ? string2 : string1 + "、" + string2
})
// $0表示計(jì)算后的結(jié)果, $1表示數(shù)組中的每一個(gè)元素
strArray1.reduce("", {
? ? return $0 == "" ? $1 : $0 + "彪见、" + $1
})
