[iOS] Learning note: Swift new features - constant, variable, control and function

* Use let to make a constant and var to make a variable

* Use if and let together to work with values that might be missing. These values are represented as optionals. An optional value either contains a value or contains nil to indicate that the value is missing. Write a question mark (?) after the type of a value to mark the value as optional

• var optionalString: String? = "Hello"
• optionalString == nil
• var optionalName: String? = "John Appleseed"
• var greeting = "Hello!"
• if let name = optionalName {
• greeting = "Hello, \(name)"
• }

* Switch don't need "break" any more

• let vegetable = "red pepper"
• switch vegetable {
• case "celery":
• let vegetableComment = "Add some raisins and make ants on a log."
• case "cucumber", "watercress":
• let vegetableComment = "That would make a good tea sandwich."
• case let x where x.hasSuffix("pepper"):
• let vegetableComment = "Is it a spicy \(x)?"
• default:
• let vegetableComment = "Everything tastes good in soup."
• }

* You use for-in to iterate over items in a dictionary by providing a pair of names to use for each key-value pair

• let interestingNumbers = [
• "Prime": [2, 3, 5, 7, 11, 13],
• "Fibonacci": [1, 1, 2, 3, 5, 8],
• "Square": [1, 4, 9, 16, 25],
• ]
• var largest = 0
• for (kind, numbers) in interestingNumbers {
• for number in numbers {
• if number > largest {
• largest = number
• }
• }
• }

* Use a tuple to return multiple values from a function

• func getGasPrices() -> (Double, Double, Double) {
• return (3.59, 3.69, 3.79)
• }

*. Functions can be nested

• func returnFifteen() -> Int {
• var y = 10
• func add() {
• y += 5
• }
• add()
• return y
• }
• 


*. Function can return another function as its value

• func makeIncrementer() -> (Int -> Int) {
• func addOne(number: Int) -> Int {
• return 1 + number
• }
• return addOne
• }
• var increment = makeIncrementer()
• 


*. Function can take another function as one of its arguments

• func hasAnyMatches(list: Int[], condition: Int -> Bool) -> Bool {
• for item in list {
• if condition(item) {
• return true
• }
• }
• return false
• }
• func lessThanTen(number: Int) -> Bool {
• return number < 10
• }
• var numbers = [20, 19, 7, 12]
• hasAnyMatches(numbers, lessThanTen)
• 


*. Functions are actually a special case of closures. You can write a closure without a name by surrounding code with braces ({}). Use in to separate the arguments and return type from the body

• numbers.map({
• (number: Int) -> Int in
• let result = 3 * number
• return result
• })
• or
• numbers.map({ number in 3 * number })
• 


*. You can refer to parameters by number instead of by name

• sort([1, 5, 3, 12, 2]) { $0 > $1 }
•