Basic knowledge

Declare variables or constants

Variables are defined with var and constants are defined with let.

var myVariable = 42
myVariable = 50
let myConstant = 42

When the type variable or constant type is not specified, the system automatically infers the type of the variable or constant. If the type==myVariable=== is inferred as an integer.

Specified type

Use a colon when specifying a type.

let explicitDouble: Double = 70

Type conversion

let label = "The width is "
let width = 94
let widthLabel = label + String(width)

String(width) converts an integer to a string.

let apples = 3
let oranges = 5
let appleSummary = "I have \(apples) apples."
let fruitSummary = "I have \(apples + oranges) pieces of fruit."

\ () Embedding variables into strings.

let quotation = """
I said "I have \(apples) apples."
And then I said "I have \(apples + oranges) pieces of fruit."
"""

== ""== Multi-line nested string.

array

Create an array and modify the value of the corresponding index of the array by index.

var shoppingList = ["catfish", "water", "tulips", "blue paint"]
shoppingList[1] = "bottle of water"

Dictionaries

Create a dictionary and modify the corresponding values

var occupations = [
    "Malcolm": "Captain",
    "Kaylee": "Mechanic",
]
occupations["Jayne"] = "Public Relations"

Define empty arrays and dictionaries

let emptyArray = [String]()
let emptyDictionary = [String: Float]()

control flow

Use if and switch as conditions, use for-in, while, and repeat-while as loops.

let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0
for score in individualScores {
    if score > 50 {
        teamScore += 3
    } else {
        teamScore += 1
    }
}
print(teamScore)

Optional value

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

Represents that optionalString can be nil.

let nickName: String? = nil
let fullName: String = "John Appleseed"
let informalGreeting = "Hi \(nickName ?? fullName)"

NickName?? fullName: If nickName is nil, output fullName, otherwise output nickName.

switch

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

The case condition of switch must have=== default===, otherwise compile error.

for-in

for-in traverses dictionaries, and arrays.

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
        }
    }
}
print(largest)

0. <4: A range of 0 to 4 (excluding 4)

var total = 0
for i in 0..<4 {
    total += i
}
print(total)

While and repeat-while

var n = 2
while n < 100 {
    n *= 2
}
print(n)

var m = 2
repeat {
    m *= 2
} while m < 100
print(m)

Functions and Closures

Declare a function with == func ==, -> separate parameters and return values

func greet(person: String, day: String) -> String {
    return "Hello \(person), today is \(day)."
}
greet(person: "Bob", day: "Tuesday")

Use what parameters have no label name.

func greet(_ person: String, on day: String) -> String {
    return "Hello \(person), today is \(day)."
}
greet("John", on: "Wednesday")

Functions can be nested

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

Functions can also return a function.

func makeIncrementer() -> ((Int) -> Int) {
    func addOne(number: Int) -> Int {
        return 1 + number
    }
    return addOne
}
var increment = makeIncrementer()
increment(7)

The parameters of a function can also be functions.

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]
let re = hasAnyMatches(list: numbers, condition: lessThanTen)

closure

Closures are functions that can read the internal variables of other functions. Generally speaking, they are internal functions, also known as anonymous functions.

In the code block {}, the function body and parameters are separated by in, and the returner

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

The parameter type and return value type of the internal function can be omitted because they can be inferred.

let mappedNumbers = numbers.map({ number in 3 * number })
print(mappedNumbers)

Even, you can use $0, $1,... To represent parameters in sequence.

let mappedNumbers = numbers.map{3 * $0}
print(mappedNumbers)