# A Swift Tour > K&R uses “hello, world”. It seems worth breaking with tradition to use proper casing. - Skill: `tools-only/a-swift-tour` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add tools-only/a-swift-tour` - Raw SKILL.md: https://api.skillmd.com/api/skills/tools-only/a-swift-tour/raw - Safety review: pending (external: skill-scanner PASS, skillspector CAUTION) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tools-only (https://skillmd.com/u/tools-only) - Updated: 2026-09-29 - Page: https://skillmd.com/skills/tools-only/a-swift-tour --- # A Swift Tour Explore the features and syntax of Swift. Tradition suggests that the first program in a new language should print the words “Hello, world!” on the screen. In Swift, this can be done in a single line: ```swift print("Hello, world!") // Prints "Hello, world!" ``` This syntax should look familiar if you know another language --- in Swift, this line of code is a complete program. You don't need to import a separate library for functionality like outputting text or handling strings. Code written at global scope is used as the entry point for the program, so you don't need a `main()` function. You also don't need to write semicolons at the end of every statement. This tour gives you enough information to start writing code in Swift by showing you how to accomplish a variety of programming tasks. Don’t worry if you don’t understand something --- everything introduced in this tour is explained in detail in the rest of this book. ## Simple Values Use `let` to make a constant and `var` to make a variable. The value of a constant doesn't need to be known at compile time, but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places. ```swift var myVariable = 42 myVariable = 50 let myConstant = 42 ``` A constant or variable must have the same type as the value you want to assign to it. However, you don't always have to write the type explicitly. Providing a value when you create a constant or variable lets the compiler infer its type. In the example above, the compiler infers that `myVariable` is an integer because its initial value is an integer. If the initial value doesn't provide enough information (or if there isn't an initial value), specify the type by writing it after the variable, separated by a colon. ```swift let implicitInteger = 70 let implicitDouble = 70.0 let explicitDouble: Double = 70 ``` > Experiment: Create a constant with > an explicit type of `Float` and a value of `4`. Values are never implicitly converted to another type. If you need to convert a value to a different type, explicitly make an instance of the desired type. ```swift let label = "The width is " let width = 94 let widthLabel = label + String(width) ``` > Experiment: Try removing the conversion to `String` from the last line. > What error do you get? There's an even simpler way to include values in strings: Write the value in parentheses, and write a backslash (`\`) before the parentheses. For example: ```swift let apples = 3 let oranges = 5 let appleSummary = "I have \(apples) apples." let fruitSummary = "I have \(apples + oranges) pieces of fruit." ``` > Experiment: Use `\()` to > include a floating-point calculation in a string > and to include someone’s name in a greeting. Use three double quotation marks (`"""`) for strings that take up multiple lines. Indentation at the start of each quoted line is removed, as long as it matches the indentation of the closing quotation marks. For example: ```swift let quotation = """ Even though there's whitespace to the left, the actual lines aren't indented. Except for this line. Double quotes (") can appear without being escaped. I still have \(apples + oranges) pieces of fruit. """ ``` Create arrays and dictionaries using brackets (`[]`), and access their elements by writing the index or key in brackets. A comma is allowed after the last element. ```swift var fruits = ["strawberries", "limes", "tangerines"] fruits[1] = "grapes" var occupations = [ "Malcolm": "Captain", "Kaylee": "Mechanic", ] occupations["Jayne"] = "Public Relations" ``` Arrays automatically grow as you add elements. ```swift fruits.append("blueberries") print(fruits) // Prints "["strawberries", "grapes", "tangerines", "blueberries"]". ``` You also use brackets to write an empty array or dictionary. For an array, write `[]`, and for a dictionary, write `[:]`. ```swift fruits = [] occupations = [:] ``` If you're assigning an empty array or dictionary to a new variable, or another place where there isn't any type information, you need to specify the type. ```swift let emptyArray: [String] = [] let emptyDictionary: [String: Float] = [:] ``` ## Control Flow Use `if` and `switch` to make conditionals, and use `for`-`in`, `while`, and `repeat`-`while` to make loops. Parentheses around the condition or loop variable are optional. Braces around the body are required. ```swift let individualScores = [75, 43, 103, 87, 12] var teamScore = 0 for score in individualScores { if score > 50 { teamScore += 3 } else { teamScore += 1 } } print(teamScore) // Prints "11". ``` In an `if` statement, the conditional must be a Boolean expression --- this means that code such as `if score { ... }` is an error, not an implicit comparison to zero. You can write `if` or `switch` after the equal sign (`=`) of an assignment or after `return`, to choose a value based on the condition. ```swift let scoreDecoration = if teamScore > 10 { "🎉" } else { "" } print("Score:", teamScore, scoreDecoration) // Prints "Score: 11 🎉". ``` You can 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 a value is missing. Write a question mark (`?`) after the type of a value to mark the value as optional. ```swift var optionalString: String? = "Hello" print(optionalString == nil) // Prints "false". var optionalName: String? = "John Appleseed" var greeting = "Hello!" if let name = optionalName { greeting = "Hello, \(name)" } ``` > Experiment: Change `optionalName` to `nil`. > What greeting do you get? > Add an `else` clause that sets a different greeting > if `optionalName` is `nil`. If the optional value is `nil`, the conditional is `false` and the code in braces is skipped. Otherwise, the optional value is unwrapped and assigned to the constant after `let`, which makes the unwrapped value available inside the block of code. Another way to handle optional values is to provide a default value using the `??` operator. If the optional value is missing, the default value is used instead. ```swift let nickname: String? = nil let fullName: String = "John Appleseed" let informalGreeting = "Hi \(nickname ?? fullName)" ``` You can use a shorter spelling to unwrap a value, using the same name for that unwrapped value. ```swift if let nickname { print("Hey, \(nickname)") } // Doesn't print anything, because nickname is nil. ``` Switches support any kind of data and a wide variety of comparison operations --- they aren't limited to integers and tests for equality. ```swift 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.") } // Prints "Is it a spicy red pepper?" ``` > Experiment: Try removing the default case. > What error do you get? Notice how `let` can be used in a pattern to assign the value that matched the pattern to a constant. After executing the code inside the switch case that matched, the program exits from the switch statement. Execution doesn't continue to the next case, so you don't need to explicitly break out of the switch at the end of each case’s code. You use `for`-`in` to iterate over items in a dictionary by providing a pair of names to use for each key-value pair. Dictionaries are an unordered collection, so their keys and values are iterated over in an arbitrary order. ```swift 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 (_, numbers) in interestingNumbers { for number in numbers { if number > largest { largest = number } } } print(largest) // Prints "25". ``` > Experiment: Replace the `_` with a variable name, > and keep track of which kind of number was the largest. Use `while` to repeat a block of code until a condition changes. The condition of a loop can be at the end instead, ensuring that the loop is run at least once. ```swift var n = 2 while n < 100 { n *= 2 } print(n) // Prints "128". var m = 2 repeat { m *= 2 } while m < 100 print(m) // Prints "128". ``` > Experiment: > Change the condition from `m < 100` to `m < 0` > to see how `while` and `repeat`-`while` behave differently > when the loop condition is already false. You can keep an index in a loop by using `..<` to make a range of indexes. ```swift var total = 0 for i in 0..<4 { total += i } print(total) // Prints "6". ``` Use `..<` to make a range that omits its upper value, and use `...` to make a range that includes both values. ## Functions and Closures Use `func` to declare a function. Call a function by following its name with a list of arguments in parentheses. Use `->` to separate the parameter names and types from the function's return type. ```swift func greet(person: String, day: String) -> String { return "Hello \(person), today is \(day)." } greet(person: "Bob", day: "Tuesday") ``` > Experiment: Remove the `day` parameter. > Add a parameter to include today’s lunch special in the greeting. By default, functions use their parameter names as labels for their arguments. Write a custom argument label before the parameter name, or write `_` to use no argument label. ```swift func greet(_ person: String, on day: String) -> String { return "Hello \(person), today is \(day)." } greet("John", on: "Wednesday") ``` Use a tuple to make a compound value --- for example, to return multiple values from a function. The elements of a tuple can be referred to either by name or by number. ```swift func calculateStatistics(scores: [Int]) -> (min: Int, max: Int, sum: Int) { var min = scores[0] var max = scores[0] var sum = 0 for score in scores { if score > max { max = score } else if score < min { min = score } sum += score } return (min, max, sum) } let statistics = calculateStatistics(scores: [5, 3, 100, 3, 9]) print(statistics.sum) // Prints "120". print(statistics.2) // Prints "120". ``` Functions can be nested. Nested functions have access to variables that were declared in the outer function. You can use nested functions to organize the code in a function that's long or complex. ```swift func returnFifteen() -> Int { var y = 10 func add() { y += 5 } add() return y } returnFifteen() ``` Functions are a first-class type. This means that a function can return another function as its value. ```swift func makeIncrementer() -> ((Int) -> Int) { func addOne(number: Int) -> Int { return 1 + number } return addOne } var increment = makeIncrementer() increment(7) ``` A function can take another function as one of its arguments. ```swift 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(list: numbers, condition: lessThanTen) ``` Functions are actually a special case of closures: blocks of code that can be called later. The code in a closure has access to things like variables and functions that were available in the scope where the closure was created, even if the closure is in a different scope when it's executed --- you saw an example of this already with nested functions. 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. ```swift numbers.map({ (number: Int) -> Int in let result = 3 * number return result }) ``` > Experiment: Rewrite the closure to return zero for all odd numbers. You have several options for writing closures more concisely. When a closure's type is already known, such as the callback for a delegate, you can omit the type of its parameters, its return type, or both. Single statement closures implicitly return the value of their only statement. ```swift let mappedNumbers = numbers.map({ number in 3 * number }) print(mappedNumbers) // Prints "[60, 57, 21, 36]". ``` You can refer to parameters by number instead of by name --- this approach is especially useful in very short closures. A closure passed as the last argument to a function can appear immediately after the parentheses. When a closure is the only argument to a function, you can omit the parentheses entirely. ```swift let sortedNumbers = numbers.sorted { $0 > $1 } print(sortedNumbers) // Prints "[20, 19, 12, 7]". ``` ## Objects and Classes Use `class` followed by the class's name to create a class. A property declaration in a class is written the same way as a constant or variable declaration, except that it's in the context of a class. Likewise, method and function declarations are written the same way. ```swift class Shape { var numberOfSides = 0 func simpleDescription() -> String { return "A shape with \(numberOfSides) sides." } } ``` > Experiment: Add a constant property with `let`, > and add another method that takes an argument. Create an instance of a class by putting parentheses after the class name. Use dot syntax to access the properties and methods of the instance. ```swift var shape = Shape() shape.numberOfSides = 7 var shapeDescription = shape.simpleDescription() ``` This version of the `Shape` class is missing something important: an initializer to set up the class when an instance is created. Use `init` to create one. ```swift class NamedShape { var numberOfSides: Int = 0 var name: String init(name: String) { self.name = name } func simpleDescription() -> String { return "A shape with \(numberOfSides) sides." } } ``` Notice how `self` is used to distinguish the `name` property from the `name` argument to the initializer. The arguments to the initializer are passed like a function call when you create an instance of the class. Every property needs a value assigned --- either in its declaration (as with `numberOfSides`) or in the initializer (as with `name`). Use `deinit` to create a deinitializer if you need to perform some cleanup before the object is deallocated. Subclasses include their superclass name after their class name, separated by a colon. There's no requirement for classes to subclass any standard root class, so you can include or omit a superclass as needed. Methods on a subclass that override the superclass's implementation are marked with `override` --- overriding a method by accident, without `override`, is detected by the compiler as an error. The compiler also detects methods with `override` that don't actually override any method in the superclass. ```swift class Square: NamedShape { var sideLength: Double init(sideLength: Double, name: String) { self.sideLength = sideLength super.init(name: name) numberOfSides = 4 } func area() -> Double { return sideLength * sideLength } override func simpleDescription() -> String { return "A square with sides of length \(sideLength)." } } let test = Square(sideLength: 5.2, name: "my test square") test.area() test.simpleDescription() ``` > Experiment: Make another subclass of `NamedShape` > called `Circle` > that takes a radius and a name > as arguments to its initializer. > Implement an `area()` and a `simpleDescription()` method > on the `Circle` class. In addition to simple properties that are stored, properties can have a getter and a setter. ```swift class EquilateralTriangle: NamedShape { var sideLength: Double = 0.0 init(sideLength: Double, name: String) { self.sideLength = sideLength super.init(name: name) numberOfSides = 3 } var perimeter: Double { get { return 3.0 * sideLength } set { sideLength = newValue / 3.0 } } override func simpleDescription() -> String { return "An equilateral triangle with sides of length \(sideLength)." } } var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle") print(triangle.perimeter) // Prints "9.3". triangle.perimeter = 9.9 print(triangle.sideLength) // Prints "3.3000000000000003". ``` In the setter for `perimeter`, the new value has the implicit name `newValue`. You can provide an explicit name in parentheses after `set`. Notice that the initializer for the `EquilateralTriangle` class has three different steps: 1. Setting the value of properties that the subclass declares. 2. Calling the superclass's initializer. 3. Changing the value of properties defined by the superclass. Any additional setup work that uses methods, getters, or setters can also be done at this point. If you don't need to compute the property but still need to provide code that's run before and after setting a new value, use `willSet` and `didSet`. The code you provide is run any time the value changes outside of an initializer. For example, the class below ensures that the side length of its triangle is always the same as the side length of its square. ```swift class TriangleAndSquare { var triangle: EquilateralTriangle { willSet { square.sideLength = newValue.sideLength } } var square: Square { willSet { triangle.sideLength = newValue.sideLength } } init(size: Double, name: String) { square = Square(sideLength: size, name: name) triangle = EquilateralTriangle(sideLength: size, name: name) } } var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape") print(triangleAndSquare.square.sideLength) // Prints "10.0". print(triangleAndSquare.triangle.sideLength) // Prints "10.0". triangleAndSquare.square = Square(sideLength: 50, name: "larger square") print(triangleAndSquare.triangle.sideLength) // Prints "50.0". ``` When working with optional values, you can write `?` before operations like methods, properties, and subscripting. If the value before the `?` is `nil`, everything after the `?` is ignored and the value of the whole expression is `nil`. Otherwise, the optional value is unwrapped, and everything after the `?` acts on the unwrapped value. In both cases, the value of the whole expression is an optional value. ```swift let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square") let sideLength = optionalSquare?.sideLength ``` ## Enumerations and Structures Use `enum` to create an enumeration. Like classes and all other named types, enumerations can have methods associated with them. ```swift enum Rank: Int { case ace = 1 case two, three, four, five, six, seven, eight, nine, ten case jack, queen, king func simpleDescription() -> String { switch self { case .ace: return "ace" case .jack: return "jack" case .queen: return "queen" case .king: return "king" default: return String(self.rawValue) } } } let ace = Rank.ace let aceRawValue = ace.rawValue ``` > Experiment: Write a function that compares two `Rank` values > by comparing their raw values. By default, Swift assigns the raw values starting at zero and incrementing by one each time, but you can change this behavior by explicitly specifying values. In the example above, `Ace` is explicitly given a raw value of `1`, and the rest of the raw values are assigned in order. You can also use strings or floating-point numbers as the raw type of an enumeration. Use the `rawValue` property to access the raw value of an enumeration case. Use the `init?(rawValue:)` initializer to make an instance of an enumeration from a raw value. It returns either the enumeration case matching the raw value or `nil` if there's no matching `Rank`. ```swift if let convertedRank = Rank(rawValue: 3) { let threeDescription = convertedRank.simpleDescription() } ``` The case values of an enumeration are actual values, not just another way of writing their raw values. In fact, in cases where there isn't a meaningful raw value, you don't have to provide one. ```swift enum Suit { case spades, hearts, diamonds, clubs func simpleDescription() -> String { switch self { case .spades: return "spades" case .hearts: return "hearts" case .diamonds: return "diamonds" case .clubs: return "clubs" } } } let hearts = Suit.hearts let heartsDescription = hearts.simpleDescription() ``` > Experiment: Add a `color()` method to `Suit` that returns "black" > for spades and clubs, and returns "red" for hearts and diamonds. Notice the two ways that the `hearts` case of the enumeration is referred to above: When assigning a value to the `hearts` constant, the enumeration case `Suit.hearts` is referred to by its full name because the constant doesn't have an explicit type specified. Inside the switch, the enumeration case is referred to by the abbreviated form `.hearts` because the value of `self` is already known to be a suit. You can use the abbreviated form anytime the value's type is already known. If an enumeration has raw values, those values are determined as part of the declaration, which means every instance of a particular enumeration case always has the same raw value. Another choice for enumeration cases is to have values associated with the case --- these values are determined when you make the instance, and they can be different for each instance of an enumeration case. You can think of the associated values as behaving like stored properties of the enumeration case instance. For example, consider the case of requesting the sunrise and sunset times from a server. The server either responds with the requested information, or it responds with a description of what went wrong.