基本操作

• 賦值運算

//賦值運算符(a = b)用b的值初始化或更新a的值:
let b = 10
var a = 5
a = b
// a is now equal to 10

//如果賦值的右邊是一個具有多個值的元組，那麼它的元素可以同時分解爲多個常量或變量:
let (x,y) = (1,2)
// x is equal to 1, and y is equal to 2

//與C和Objective-C中的賦值操作符不同，Swift中的賦值操作符本身並不返回值。下列表達式無效:
if x = y {
    // This is not valid, because x = y does not return a value.
}

//該特性可以防止在實際使用equal to操作符(==)時意外使用賦值操作符(=)。通過使if x = y無效，Swift可以幫助您避免代碼中的此類錯誤。

• 算數運算
  Swift支持所有數字類型的四種標準算數運算。加減乘除

 1 + 2       // equals 3
 5 - 3       // equals 2
 2 * 3       // equals 6
 10.0 / 2.5  // equals 4.0

• 求餘操作

//餘數運算符(a % b)計算出b在a中的倍數，並返回剩餘的值(稱爲餘數)。
9 % 4    // equals 1
-9 % 4    // equals 1

• 一元操作

//一元減操作 數值的符號可以使用前綴-來切換
let three = 3
let minusThree = -three       // minusThree equals -3
let plusThree = -minusThree   // plusThree equals 3, or "minus minus three"

//一元加操作符 簡單地返回它操作的值，沒有任何變化。
let minusSix = -6
let alsoMinusSix = +minusSix  
//雖然一元加號運算符實際上什麼都不做，但是當你對負數也使用一元減號運算符時，你可以用它在你的代碼中爲正數提供對稱性。

• 複合賦值操作

var c = 1
c += 2
// a is now equal to 3
//表達式a += 2是a = a + 2的簡寫。實際上，加法和賦值被合併到一個操作符中，該操作符同時執行兩個任務。

• 比較操作

/*
 Equal to (a == b)
 Not equal to (a != b)
 Greater than (a > b)
 Less than (a < b)
 Greater than or equal to (a >= b)
 Less than or equal to (a <= b)
 
 Swift also provides two identity operators (=== and !==), which you use to test whether two object references both refer to the same object instance.
 */
1 == 1   // true because 1 is equal to 1
2 != 1   // true because 2 is not equal to 1
2 > 1    // true because 2 is greater than 1
1 < 2    // true because 1 is less than 2
1 >= 1   // true because 1 is greater than or equal to 1
2 <= 1   // false because 2 is not less than or equal to 1
//Comparison operators are often used in conditional statements, such as the if statement:
let name = "world"
if name == "world" {
    print("hello, world")
} else {
    print("I'm sorry \(name), but I don't recognize you")
}

(1, "zebra") < (2, "apple")   // true because 1 is less than 2; "zebra" and "apple" are not compared
(3, "apple") < (3, "bird")    // true because 3 is equal to 3, and "apple" is less than "bird"
(4, "dog") == (4, "dog")      // true because 4 is equal to 4, and "dog" is equal to "dog"

• 三元條件操作

let contentHeight = 40
let hasHeader = true
let rowHeight = contentHeight + (hasHeader ? 50 : 20)

等同於：
let contentHeight = 40
let hasHeader = true
let rowHeight: Int
if hasHeader {
    rowHeight = contentHeight + 50
} else {
    rowHeight = contentHeight + 20
}

• Nil Coalescing運算

(a ?? b)
 1. a必須是Optional類型的。
 2. b的類型必須要和a解包後的值類型一致
 let defaultColorName = "red"
var userDefinedColorName: String?   // defaults to nil

var colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is nil, so colorNameToUse is set to the default of "red"

userDefinedColorName = "green"
colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is not nil, so colorNameToUse is set to "green"

• 範圍操作

//閉合範圍操作 (a...b)
for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

//半開範圍操作 (a..<b)
let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..<count {
    print("Person \(i + 1) is called \(names[i])")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack

//單邊操作
for name in names[2...] {
    print(name)
}
// Brian
// Jack

for name in names[...2] {
    print(name)
}
// Anna
// Alex
// Brian

//半開單邊操作
for name in names[..<2] {
    print(name)
}
// Anna
// Alex

let range = ...5
range.contains(7)   // false
range.contains(4)   // true
range.contains(-1)  // true

• 邏輯操作
  Logical NOT (!a)
  Logical AND (a && b)
  Logical OR (a || b)

//非操作
let allowedEntry = false
if !allowedEntry {
    print("ACCESS DENIED")
}
// Prints "ACCESS DENIED"

//與操作 只有當兩個表達式爲真時，整個表達式才爲真。如果其中一個表達式爲false，那麼整個表達式也爲false。如果第一個表達式爲false，第二個表達式將不會被求值
let enteredDoorCode = true
let passedRetinaScan = false
if enteredDoorCode && passedRetinaScan {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "ACCESS DENIED"

//或操作  表達式有一個爲真時，整個表達式爲真。如果第一個表達式爲真，將不會對第二個表達式進行求值，因爲整個表達式爲真
let hasDoorKey = false
let knowsOverridePassword = true
if hasDoorKey || knowsOverridePassword {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "Welcome!"

//組合邏輯操作
if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "Welcome!"

//可以添加圓括號，使表達式更加清晰
if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "Welcome!