习题笔记

今天解决一下 Higher Order Functions: Map,Filter,Reduce and more – Part 1 中最后的13个练习题，主要是关于闭包的（咦不对啊我明明是想看集合方面内容的。。。）。

Write a function applyTwice(f:(Float -> Float),x:Float) -> Float that takes a function f and a float x and aplies f to x twice i.e. f(f(x))

func applyTwice(f:(Float -> Float),x:Float) -> Float {
    return f(f(x))
}

Write a function applyKTimes(f:(Float -> Float),x:Float,k:Int) -> Float that takes a function f and a float x and aplies f to x k times

// recursive version
func applyKTimes(f:(Float -> Float),k:Int) -> Float
{
    return k > 0 ? applyKTimes(f,f(x),k - 1) : x
}

// unrolled by hand
func applyKTimes(f:(Float -> Float),k:Int) -> Float {
    var y : Float = x
    for _ in 0..<k {
        y = f(y)
    }
    return y
}

Using applyKTimes write a function that raises x to the kth power

func getKthPower(x:Float,k:Int) -> Float{
    return applyKTimes( {x * $0},1,k) // {x * $0} => {p in return x * p}
}
getKthPower(2.0,3) // 8.0

Given an array of Users which have properties name:String and age:Int write a map function that returns an array of strings consisting of the user’s names

class User {
    var name: String?
    var age : Int?

    init (name: String,age:Int) {
        self.name = name
        self.age = age
    }
}

var user1 = User(name: "WHY1",age: 22)
var user2 = User(name: "WHY2",age: 23)
var user3 = User(name: "WHY3",age: 24)
var user4 = User(name: "WHY4",age: 25)
var users = [user1,user2,user3,user4]
var names = users.map({(user:User) in user.name!})
println(names) // [WHY1,WHY2,WHY3,WHY4]

Given an array of of dictionaries containing keys for “name” and “age” write a map function that returns an array of users created from it

var users = [["name":"WHY1","age":"22"],["name":"WHY2","age":"23"],["name":"WHY3","age":"24"],["name":"WHY4","age":"25"]]
var result = users.map({ (userDic:[String:String]) -> User  in
    return User(name: userDic["name"]!,age:userDic["age"]!.toInt()!)

})

Given an array of numbers write a filter method that only selects odd integers

var nums = [1,2,4,8,23,45,89,127]
var odds = nums.filter({ $0 % 2 == 0 }) // 2 4 8

Given an array of strings write a filter function that selects only strings that can be converted to Ints

var strs = ["2333","1223","callmewhy","callherhh"]
var intables = strs.filter({ $0.toInt() != nil }) // ["2333","1223"]

Given an array of UIViews write a filter function that selects only those views that are a subclass of UILabel

import UIKit

var view1 = UIView()
var view2 = UIView()
var view3 = UILabel()
var view4 = UIView()
var views = [view1,view2,view3,view4]
var labels = views.filter({ $0.isKindOfClass(UILabel) }) // view3

Write a reduce function that takes an array of strings and returns a single string consisting of the given strings separated by newlines

var strs = ["str1","str2","str3","str4"]
var str = strs.reduce("",combine: { "($0)n($1)" })
println(str)

Write a reduce function that finds the largest element in an array of Ints

var ints = [1,3,5,6]
var maxValue = ints.reduce(Int.min,{ max($0,$1) })    // 6

You could implement a mean function using the reduce operation {$0 +$1 / Float(array.count)}. Why is this a bad idea?

var array = [1,6]
var mean = array.reduce(0,combine: {$0 + Float($1) / Float(array.count)}) // make division 5 times

There’s a problem you encounter when trying to implement a parallel version of reduce. What property should the operation have to make this easier ?

// TODO

Implement Church Numerals in Swift (This is a difficult and open ended exercise)

// TODO

References

• Higher Order Functions: Map,Reduce and more – Part 1
• How to write a function applies f to x k times?
• Raise x to the Kth power?
• Correct way to find max in an Array in Swift
• Swift equivalent for MIN and MAX macros