函数式编程学习

Swift Functional Programming Tutorial

烧脑体操（三）
烧脑体操（四）

这几天学习了一下函数式编程，简直是思维上的颠覆，代码可以这样思考。

这篇文章简略的Filter和reduce实现

func myFilter<T>(source: [T], predicate:(T) -> Bool) -> [T] {
    var result = [T]()
    for i in source {
        if predicate(i) {
            result.append(i)
        }
    }
    return result
}

extension Array {
    func myReduce<T, U>(seed:U, combiner:(U, T) -> U) -> U {
        var current = seed
        for item in self {
            current = combiner(current, item as T)
        }
        return current
    }
}

最后给出一个例子

let words = ["Cat", "Chicken", "fish", "Dog",
                      "Mouse", "Guinea Pig", "monkey"]

实现对单词的首字母归类
[("C", ["Cat", "Chicken"]), ("F", ["fish"]), ("D", ["Dog"]), ("M", ["Mouse", "monkey"]), ("G", ["Guinea Pig"])]

我给出的常规的实现

typealias Entry = (Character, [String])

func buildIndex(words:[String]) -> [Entry]{
    
    var entryList = [Entry]()
    
    for word in words {
        let firstLetter = word.uppercaseString[word.startIndex]
        
        var find = false
        for (index, entry) in entryList.enumerate() {
            if firstLetter == entry.0 {
                find = true
                /*
                entry.1.append(word)
                //重点！！一定要记住值类型和引用类型
                //entry是值类型
                entryList[index] = entry
                */
                //优化
                entryList[index].1.append(word)
            }
        }
        
        if !find {
            let entry : (Character, [String]) = (firstLetter, [word])
            entryList.append(entry)
        }
    }
    
    return entryList
}

改为函数式编程的实现

let compute = words.reduce([]) { (var arr, word) -> [Character] in
    let letter = word.uppercaseString[word.startIndex]
    if !arr.contains(letter) {
        arr.append(letter)
    }
    return arr
}.map { (letter) -> Entry in
    return (letter, words.filter({ (word) -> Bool in
        return word.uppercaseString[word.startIndex] == letter
    }))
}

觉得比文章的实现要好