let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0

var x = 0.0, y = 0.0, z = 0.0

var welcomeMessage: String
 
var red, green, blue: Double

// This is a comment.
 
/* This is also a comment
 but is written over multiple lines. */
 
/* This is the start of the first multiline comment.
 /* This is the second, nested multiline comment. */
 This is the end of the first multiline comment. */

let cat = "??"; print(cat)

let decimalInteger = 17
let binaryInteger = 0b10001       // 17 二进制
let octalInteger = 0o21           // 17 八进制
let hexadecimalInteger = 0x11     // 17 十六进制

let decimalDouble = 12.1875
let exponentDouble = 1.21875e1  //科学计数法 1.21875*10
let hexadecimalDouble = 0xC.3p0 // p0代表 2的0次方

let paddedDouble = 000123.456
let oneMillion = 1_000_000
let justOverOneMillion = 1_000_000.000_000_1

let three = 3
let pointOneFourOneFiveNine = 0.14159
let pi = Double(three) + pointOneFourOneFiveNine

let orangesAreOrange = true
let turnipsAreDelicious = false

if turnipsAreDelicious {
    print("Mmm, tasty turnips!")
} else {
    print("Eww, turnips are horrible.")
}

let http404Error = (404, "Not Found")
// http404Error is of type (Int, String), and equals (404, "Not Found")

let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)")
// Prints "The status code is 404"
print("The status message is \(statusMessage)")
// Prints "The status message is Not Found"

let (justTheStatusCode, _) = http404Error
print("The status code is \(justTheStatusCode)")

print("The status code is \(http404Error.0)")
// Prints "The status code is 404"
print("The status message is \(http404Error.1)")
// Prints "The status message is Not Found"

var serverResponseCode: Int? = 404
// serverResponseCode contains an actual Int value of 404
serverResponseCode = nil
// serverResponseCode now contains no value

if convertedNumber != nil {
    print("convertedNumber has an integer value of \(convertedNumber!).")
}

if let constantName = someOptional {
    statements
}

if let actualNumber = Int(possibleNumber) {
    print("\"\(possibleNumber)\" has an integer value of \(actualNumber)")
} else {
    print("\"\(possibleNumber)\" could not be converted to an integer")
}
// Prints ""123" has an integer value of 123"

if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
    print("\(firstNumber) < \(secondNumber) < 100")
}
// Prints "4 < 42 < 100"
 
if let firstNumber = Int("4") {
    if let secondNumber = Int("42") {
        if firstNumber < secondNumber && secondNumber < 100 {
            print("\(firstNumber) < \(secondNumber) < 100")
        }
    }
}
// Prints "4 < 42 < 100"

let possibleString: String? = "An optional string."
let forcedString: String = possibleString! // requires an exclamation mark
 
let assumedString: String! = "An implicitly unwrapped optional string."
let implicitString: String = assumedString // no need for an exclamation mark

if let definiteString = assumedString {
    print(definiteString)
}
// Prints "An implicitly unwrapped optional string."

func canThrowAnError() throws {
    // this function may or may not throw an error
}

do {
    try canThrowAnError()
    // no error was thrown
} catch {
    // an error was thrown
}

func makeASandwich() throws {
    // ...
}
 
do {
    try makeASandwich()
    eatASandwich()
} catch SandwichError.outOfCleanDishes {
    washDishes()
} catch SandwichError.missingIngredients(let ingredients) {
    buyGroceries(ingredients)
}

let age = -3
assert(age >= 0, "A person's age can't be less than zero.")
// This assertion fails because -3 is not >= 0.

if age > 10 {
    print("You can ride the roller-coaster or the ferris wheel.")
} else if age > 0 {
    print("You can ride the ferris wheel.")
} else {
    assertionFailure("A person's age can't be less than zero.")
}

// In the implementation of a subscript...
precondition(index > 0, "Index must be greater than zero.")