Playing with Map and FlatMap

FlatMap.swift

// map function
let array = [1,2,3,4,5]  
let doubleArray = array.map { $0 * 2 }      //produces [2,4,6,8,10]

let players = ["Gerrard", "Dalglish", "Barnes"]

//Create a function upper which takes a String and returns an uppercased String
func upper(name: String) -> String {
    return name.uppercased()
}

// Pass this function into the map function
players.map(upper)                                //produces ["GERRARD", "DALGLISH", "BARNES"]

// Can pass it in as a closure expression (Note: The start of the closure’s body is introduced by the in keyword) This keyword indicates that the definition of the closure’s parameters and return type has finished, and the body of the closure is about to begin.

players.map({
    (name: String) -> String in return name.uppercased()
})                                                              //produces ["GERRARD", "DALGLISH", "BARNES"]

// as the closure is the last argment we can remove the ( brackets

players.map {
    (name: String) -> String in return name.uppercased()
}                                                                 //produces ["GERRARD", "DALGLISH", "BARNES"]
//It is always possible to infer the parameter types and return type when passing a closure to a function or method as an inline closure expression.
//Can replace (name: String) -> String with name as Swift knows it is taking a String and producing a String
players.map { name in name.uppercased() }                     //produces ["GERRARD", "DALGLISH", "BARNES"]

//Swift provides shorthand arguments so we can replace name with $0
players.map { $0.uppercased() }                             //produces ["GERRARD", "DALGLISH", "BARNES"]



//FlatMap

let liverpoolLegends = players
let liverpoolPlayers = ["Rush", "Fowler", "Owen"]


let liverpool = [liverpoolLegends, liverpoolPlayers]    //produces [["Gerrard", "Dalglish", "Barnes"], ["Rush", "Fowler", "Owen"]]

liverpool.flatMap { $0 })                                   //produces ["Gerrard", "Dalglish", "Barnes", "Rush", "Fowler", "Owen"]


//liverpool.flatMap{ $0.uppercased() }      
//ERROR
//Value of type '[String?]' has no member 'uppercased'
//$0 is each array, not the values of the array, so we need to map

liverpool.flatMap{ $0.map{ $0.uppercased() } }        //produces ["GERRARD", "DALGLISH", "BARNES", "RUSH", "FOWLER", "OWEN"]

// Also used to "magically" remove optional values

let strings = ["Hello", "World", nil, "Goodbye", nil]
strings.flatMap { $0 }                                    //produces ["Hello", "World", "Goodbye"]
strings.flatMap { $0?.uppercased() }                      //produces ["HELLO", "WORLD", "GOODBYE"]
//need to use the ? as $0 is optional