## Generics

For example, Swift’s `Array` and `Dictionary` types are both generic collections. You can create an array that holds `Int` values, or an array that holds `String` values, or indeed an array for any other type that can be created in Swift. Similarly, you can create a dictionary to store values of any specified type, and there are no limitations on what that type can be.

## The Problem That Generics Solve

Here’s a standard, nongeneric function called `swapTwoInts(_:_:)`, which swaps two `Int` values:

func swapTwoInts(_ a: inout Int, _ b: inout Int) {
let temporaryA = a
a = b
b = temporaryA
}

var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt, &anotherInt)
print(“someInt is now (someInt), and anotherInt is now (anotherInt)”)
// Prints “someInt is now 107, and anotherInt is now 3”

The `swapTwoInts(_:_:)` function is useful, but it can only be used with `Int` values. If you want to swap two `String` values, or two `Double` values, you have to write more functions, such as the `swapTwoStrings(_:_:)` and `swapTwoDoubles(_:_:)` functions shown below:

func swapTwoStrings(_ a: inout String, _ b: inout String) {
let temporaryA = a
a = b
b = temporaryA
}

func swapTwoDoubles(_ a: inout Double, _ b: inout Double) {
let temporaryA = a
a = b
b = temporaryA
}

It’s more useful, and considerably more flexible, to write a single function that swaps two values of any type. Generic code enables you to write such a function. (A generic version of these functions is defined below.)

## Generic Functions

func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
let temporaryA = a
a = b
b = temporaryA
}

1. `func swapTwoInts(_ a: inout Int, _ b: inout Int)`
2. `func swapTwoValues<T>(_ a: inout T, _ b: inout T)`

The generic version of the function uses a placeholder type name (called `T`, in this case) instead of an actual type name (such as `Int`, `String`, or `Double`). The placeholder type name doesn’t say anything about what `T `must be, but it does say that both `a` and `b` must be of the same type `T`, whatever `T` represents. The actual type to use in place of `T` is determined each time the `swapTwoValues(_:_:)` function is called.

var someInt = 3

var anotherInt = 107
swapTwoValues(&someInt, &anotherInt)
// someInt is now 107, and anotherInt is now 3
var someString = “hello”
var anotherString = “world”
swapTwoValues(&someString, &anotherString)
// someString is now “world”, and anotherString is now “hello”

## Type Parameters

In the `swapTwoValues(_:_:)` example above, the placeholder type `T` is an example of a type parameter. Type parameters specify and name a placeholder type, and are written immediately after the function’s name, between a pair of matching angle brackets (such as `<T>`).

Once you specify a type parameter, you can use it to define the type of a function’s parameters (such as the `a` and `b` parameters of the `swapTwoValues(_:_:)` function), or as the function’s return type, or as a type annotation within the body of the function.

You can provide more than one type parameter by writing multiple type parameter names within the angle brackets, separated by commas.

## Naming Type Parameters

In most cases, type parameters have descriptive names, such as `Key` and `Value` in `Dictionary<Key, Value>`and `Element` in `Array<Element>`, which tells the reader about the relationship between the type parameter and the generic type or function it’s used in. However, when there isn’t a meaningful relationship between them, it’s traditional to name them using single letters such as `T`, `U`, and `V`.

NOTE

Always give type parameters upper camel case names (such as `T` and `MyTypeParameter`

## Generic Types

In addition to generic functions, Swift enables you to define your own generic types. These are custom classes, structures, and enumerations that can work with any type, in a similar way to `Array` and `Dictionary`.

The illustration below shows the push and pop behavior for a stack:

1. There are currently three values on the stack.
2. A fourth value is pushed onto the top of the stack.
3. The stack now holds four values, with the most recent one at the top.
4. The top item in the stack is popped.
5. After popping a value, the stack once again holds three values.

Here’s how to write a nongeneric version of a stack, in this case for a stack of `Int` values:

struct IntStack {
var items = [Int]()
mutating func push(_ item: Int) {
items.append(item)
}
mutating func pop() -> Int {
return items.removeLast()
}
}

This structure uses an `Array` property called `items` to store the values in the stack. `Stack` provides two methods, `push` and `pop`, to push and pop values on and off the stack. These methods are marked as `mutating`, because they need to modify (or mutate) the structure’s `items` array.

The `IntStack` type shown above can only be used with `Int` values, however. It would be much more useful to define a generic `Stack` class, that can manage a stack of any type of value.

Here’s a generic version of the same code:

struct Stack<Element> {
var items = [Element]()
mutating func push(_ item: Element) {
items.append(item)
}
mutating func pop() -> Element {
return items.removeLast()
}
}

var stackOfStrings = Stack<String>()
stackOfStrings.push(“uno”)
stackOfStrings.push(“dos”)
stackOfStrings.push(“tres”)
stackOfStrings.push(“cuatro”)
// the stack now contains 4 strings

let fromTheTop = stackOfStrings.pop()
// fromTheTop is equal to “cuatro”, and the stack now contains 3 strings

## Extending a Generic Type

When you extend a generic type, you don’t provide a type parameter list as part of the extension’s definition. Instead, the type parameter list from the original type definition is available within the body of the extension, and the original type parameter names are used to refer to the type parameters from the original definition.

(예를 들어 아래 예시에서 Element가 type parameter 이며 이것이 extending하는 부분에서 접근가능하다는 이야기)

struct Stack<Element> {
}

extension Stack {

// Element가 extended 정의의 type parameter이므로 접근 가능
var topItem: Element? {
return items.isEmpty ? nil : items[items.count – 1]
}
}

if let topItem = stackOfStrings.topItem {
print(“The top item on the stack is (topItem).”)
}
// Prints “The top item on the stack is tres.”

## Type Constraints

Type constraints specify that a type parameter must inherit from a specific class, or conform to a particular protocol or protocol composition.

For example, Swift’s `Dictionary` type places a limitation on the types that can be used as keys for a dictionary. As described in Dictionaries, the type of a dictionary’s keys must be hashable. That is, it must provide a way to make itself uniquely representable. `Dictionary` needs its keys to be hashable so that it can check whether it already contains a value for a particular key. Without this requirement, `Dictionary` could not tell whether it should insert or replace a value for a particular key, nor would it be able to find a value for a given key that is already in the dictionary.

This requirement is enforced by a type constraint on the key type for `Dictionary`, which specifies that the key type must conform to the `Hashable` protocol, a special protocol defined in the Swift standard library. All of Swift’s basic types (such as `String`, `Int`, `Double`, and `Bool`) are hashable by default.

You can define your own type constraints when creating custom generic types, and these constraints provide much of the power of generic programming. Abstract concepts like `Hashable` characterize types in terms of their conceptual characteristics, rather than their concrete type.

## Type Constraint Syntax

You write type constraints by placing a single class or protocol constraint after a type parameter’s name, separated by a colon, as part of the type parameter list. The basic syntax for type constraints on a generic function is shown below (although the syntax is the same for generic types):

func someFunction<T: SomeClass, U: SomeProtocol>(someT: T, someU: U) {
// function body goes here
}

The hypothetical function above has two type parameters. The first type parameter, `T`, has a type constraint that requires `T` to be a subclass of `SomeClass`. The second type parameter, `U`, has a type constraint that requires `U` to conform to the protocol `SomeProtocol`.

## Type Constraints in Action

type constaints의 사용 예시

Here’s a nongeneric function called `findIndex(ofString:in:)`, which is given a `String` value to find and an array of `String` values within which to find it. The `findIndex(ofString:in:)` function returns an optional `Int`value, which will be the index of the first matching string in the array if it’s found, or `nil` if the string can’t be found:

func findIndex(ofString valueToFind: String, in array: [String]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}

The `findIndex(ofString:in:)` function can be used to find a string value in an array of strings:

let strings = [“cat”, “dog”, “llama”, “parakeet”, “terrapin”]
if let foundIndex = findIndex(ofString: “llama”, in: strings) {
print(“The index of llama is (foundIndex)”)
}
// Prints “The index of llama is 2”

The principle of finding the index of a value in an array isn’t useful only for strings, however. You can write the same functionality as a generic function by replacing any mention of strings with values of some type `T`instead.

Here’s how you might expect a generic version of `findIndex(ofString:in:)`, called `findIndex(of:in:)`, to be written. Note that the return type of this function is still `Int?`, because the function returns an optional index number, not an optional value from the array. Be warned, though—this function doesn’t compile, for reasons explained after the example:

func findIndex<T>(of valueToFind: T, in array:[T]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}

This function doesn’t compile as written above. The problem lies with the equality check, “`if value == valueToFind`”. Not every type in Swift can be compared with the equal to operator (`==`). If you create your own class or structure to represent a complex data model, for example, then the meaning of “equal to” for that class or structure isn’t something that Swift can guess for you. Because of this, it isn’t possible to guarantee that this code will work for every possible type `T`, and an appropriate error is reported when you try to compile the code.

All is not lost, however. The Swift standard library defines a protocol called `Equatable`, which requires any conforming type to implement the equal to operator (`==`) and the not equal to operator (`!=`) to compare any two values of that type. All of Swift’s standard types automatically support the `Equatable` protocol.

Any type that is `Equatable` can be used safely with the `findIndex(of:in:)` function, because it’s guaranteed to support the equal to operator. To express this fact, you write a type constraint of `Equatable` as part of the type parameter’s definition when you define the function:

func findIndex<T: Equatable>(of valueToFind: T, in array:[T]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}

The single type parameter for `findIndex(of:in:)` is written as `T: Equatable`, which means “any type `T` that conforms to the `Equatable` protocol.”

The `findIndex(of:in:)` function now compiles successfully and can be used with any type that is `Equatable`, such as `Double` or `String`:

let doubleIndex = findIndex(of: 9.3, in: [3.14159, 0.1, 0.25])
// doubleIndex is an optional Int with no value, because 9.3 isn’t in the array
let stringIndex = findIndex(of: “Andrea”, in: [“Mike”, “Malcolm”, “Andrea”])
// stringIndex is an optional Int containing a value of 2

## Associated Types

참고 사항) protocol에서 generic을 사용해야 하는 경우는 위에서 사용한 방법들과는 다르다. <T>의 방법은 사용 불가능하고 associatedtype을 대신 사용한다. 그 이전에는 typealias를 이용했으나 deprecated되었다.

참고 사항)

When defining a protocol, it’s sometimes useful to declare one or more associated types as part of the protocol’s definition. An associated type gives a placeholder name to a type that is used as part of the protocol. The actual type to use for that associated type isn’t specified until the protocol is adopted. Associated types are specified with the `associatedtype` keyword.

Associated Types in Action

Here’s an example of a protocol called `Container`, which declares an associated type called `Item`:

protocol Container {
associatedtype Item
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }
}

The `Container` protocol defines three required capabilities that any container must provide:

• It must be possible to add a new item to the container with an `append(_:)` method.
• It must be possible to access a count of the items in the container through a `count` property that returns an `Int` value.
• It must be possible to retrieve each item in the container with a subscript that takes an `Int` index value.

This protocol doesn’t specify how the items in the container should be stored or what type they’re allowed to be. The protocol only specifies the three bits of functionality that any type must provide in order to be considered a `Container`. A conforming type can provide additional functionality, as long as it satisfies these three requirements.

Any type that conforms to the `Container` protocol must be able to specify the type of values it stores. Specifically, it must ensure that only items of the right type are added to the container, and it must be clear about the type of the items returned by its subscript.

To define these requirements, the `Container` protocol needs a way to refer to the type of the elements that a container will hold, without knowing what that type is for a specific container. The `Container` protocol needs to specify that any value passed to the `append(_:)` method must have the same type as the container’s element type, and that the value returned by the container’s subscript will be of the same type as the container’s element type.

To achieve this, the `Container` protocol declares an associated type called `Item`, written as `associatedtype Item`. The protocol doesn’t define what `Item` is—that information is left for any conforming type to provide.(generic type은 protocol 에서 정의되는 것이 아니고 conform하는 부분에서 수행된다.) Nonetheless, the `Item` alias provides a way to refer to the type of the items in a `Container`, and to define a type for use with the `append(_:)` method and subscript, to ensure that the expected behavior of any `Container` is enforced.

Here’s a version of the nongeneric `IntStack` type from earlier, adapted to conform to the `Container` protocol:

struct IntStack: Container {
// original IntStack implementation
var items = [Int]()
mutating func push(_ item: Int) {
items.append(item)
}
mutating func pop() -> Int {
return items.removeLast()
}

// conformance to the Container protocol

// 아래에서 설명하겠지만 이렇게 명시적으로 하는 방법이 있지만 암묵적으로도가능하다.
typealias Item = Int

mutating func append(_ item: Int) {
self.push(item)
}
var count: Int {
return items.count
}
subscript(i: Int) -> Int {
return items[i]
}
}

The `IntStack` type implements all three of the `Container` protocol’s requirements, and in each case wraps part of the `IntStack` type’s existing functionality to satisfy these requirements.

Moreover, `IntStack` specifies that for this implementation of `Container`, the appropriate `Item` to use is a type of `Int`. The definition of `typealias Item = Int` turns the abstract type of `Item` into a concrete type of `Int` for this implementation of the `Container` protocol.

Thanks to Swift’s type inference, you don’t actually need to declare a concrete `Item` of `Int` as part of the definition of `IntStack`. Because `IntStack` conforms to all of the requirements of the `Container` protocol, Swift can infer the appropriate `Item` to use, simply by looking at the type of the `append(_:)` method’s `item`parameter and the return type of the subscript. Indeed, if you delete the `typealias Item = Int` line from the code above, everything still works, because it’s clear what type should be used for `Item`. (명시적으로    typealias Item = Int 이렇게 하지 않아도 암묵적으로 swift가 알맞은 type을 찾을수 있다.)

You can also make the generic `Stack` type conform to the `Container` protocol:

struct Stack<Element>: Container {
// original Stack<Element> implementation
var items = [Element]()
mutating func push(_ item: Element) {
items.append(item)
}
mutating func pop() -> Element {
return items.removeLast()
}
// conformance to the Container protocol
mutating func append(_ item: Element) {
self.push(item)
}
var count: Int {
return items.count
}
subscript(i: Int) -> Element {
return items[i]
}
}

This time, the type parameter `Element` is used as the type of the `append(_:)` method’s `item` parameter and the return type of the subscript. Swift can therefore infer that `Element` is the appropriate type to use as the `Item `for this particular container.

Extending an Existing Type to Specify an Associated Type

You can extend an existing type to add conformance to a protocol, as described in Adding Protocol Conformance with an Extension. This includes a protocol with an associated type.

Swift’s `Array` type already provides an `append(_:)` method, a `count` property, and a subscript with an `Int `index to retrieve its elements. These three capabilities match the requirements of the `Container` protocol. This means that you can extend `Array` to conform to the `Container` protocol simply by declaring that `Array `adopts the protocol. You do this with an empty extension, as described in Declaring Protocol Adoption with an Extension:

extension Array: Container {}

Array’s existing `append(_:)` method and subscript enable Swift to infer the appropriate type to use for `Item`, just as for the generic `Stack` type above. After defining this extension, you can use any `Array` as a `Container`.

Using Type Annotations to Constrain an Associated Type

You can add a type annotation to an associated type in a protocol, to require that conforming types satisfy the constraints described by the type annotation. For example, the following code defines a version of `Container` that requires the items in the container to be equatable.

protocol Container {

// Item 은 Equatable protocol을 conform하는 것이어야 한다.
associatedtype Item: Equatable
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }
}

To conform to this version of `Container`, the container’s `Item` type has to conform to the `Equatable` protocol.

Using Type Annotations to Constrain an Associated Type

You can add a type annotation to an associated type in a protocol, to require that conforming types satisfy the constraints described by the type annotation. For example, the following code defines a version of `Container` that requires the items in the container to be equatable.

protocol Container {
associatedtype Item: Equatable
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }
}

To conform to this version of `Container`, the container’s `Item` type has to conform to the `Equatable` protocol.

Using a Protocol in Its Associated Type’s Constraints

## (이해 못했던 부분)

A protocol can appear as part of its own requirements. For example, here’s a protocol that refines the `Container` protocol, adding the requirement of a `suffix(_:)` method. The `suffix(_:)` method returns a given number of elements from the end of the container, storing them in an instance of the `Suffix` type.

protocol SuffixableContainer: Container {
associatedtype Suffix: SuffixableContainer where Suffix.Item == Item
func suffix(_ size: Int) -> Suffix
}

In this protocol, `Suffix` is an associated type, like the `Item` type in the `Container` example above. `Suffix` has two constraints: It must conform to the `SuffixableContainer` protocol (the protocol currently being defined), and its `Item` type must be the same as the container’s `Item` type. The constraint on `Item` is a generic `where`clause, which is discussed in Associated Types with a Generic Where Clause below.

Here’s an extension of the `Stack` type from earlier that adds conformance to the `SuffixableContainer`protocol:

extension Stack: SuffixableContainer {
func suffix(_ size: Int) -> Stack {
var result = Stack()
for index in (count-size)..<count {
result.append(self[index])
}
return result
}
// Inferred that Suffix is Stack.
}
var stackOfInts = Stack<Int>()
stackOfInts.append(10)
stackOfInts.append(20)
stackOfInts.append(30)
let suffix = stackOfInts.suffix(2)
// suffix contains 20 and 30

In the example above, the `Suffix` associated type for `Stack` is also `Stack`, so the suffix operation on `Stack`returns another `Stack`. Alternatively, a type that conforms to `SuffixableContainer` can have a `Suffix` type that’s different from itself—meaning the suffix operation can return a different type. For example, here’s an extension to the nongeneric `IntStack` type that adds `SuffixableContainer` conformance, using `Stack<Int>` as its suffix type instead of `IntStack`:

extension IntStack: SuffixableContainer {
func suffix(_ size: Int) -> Stack<Int> {
var result = Stack<Int>()
for index in (count-size)..<count {
result.append(self[index])
}
return result
}
// Inferred that Suffix is Stack<Int>.
}

## Generic Where Clauses

Type constraints, as described in Type Constraints, enable you to define requirements on the type parameters associated with a generic function, subscript, or type.

It can also be useful to define requirements for associated types. You do this by defining a generic where clause. A generic `where` clause enables you to require that an associated type must conform to a certain protocol, or that certain type parameters and associated types must be the same. A generic `where` clause starts with the `where` keyword, followed by constraints for associated types or equality relationships between types and associated types. You write a generic `where` clause right before the opening curly brace of a type or function’s body.

The example below defines a generic function called `allItemsMatch`, which checks to see if two `Container`instances contain the same items in the same order. The function returns a Boolean value of `true` if all items match and a value of `false` if they don’t.

The two containers to be checked don’t have to be the same type of container (although they can be), but they do have to hold the same type of items. This requirement is expressed through a combination of type constraints and a generic `where` clause:

//

<C1: Container, C2: Container> 이부분

//

defining a generic where clause

func allItemsMatch<C1: Container, C2: Container>
(_ someContainer: C1, _ anotherContainer: C2) -> Bool
where C1.Item == C2.Item, C1.Item: Equatable {

// Check that both containers contain the same number of items.
if someContainer.count != anotherContainer.count {
return false
}

// Check each pair of items to see if they’re equivalent.
for i in 0..<someContainer.count {
if someContainer[i] != anotherContainer[i] {
return false
}
}

// All items match, so return true.
return true
}

This function takes two arguments called `someContainer` and `anotherContainer`. The `someContainer` argument is of type `C1`, and the `anotherContainer` argument is of type `C2`. Both `C1` and `C2` are type parameters for two container types to be determined when the function is called.

The following requirements are placed on the function’s two type parameters:

• `C1` must conform to the `Container` protocol (written as `C1: Container`).
• `C2` must also conform to the `Container` protocol (written as `C2: Container`).
• The `Item` for `C1` must be the same as the `Item` for `C2` (written as `C1.Item == C2.Item`).
• The `Item` for `C1` must conform to the `Equatable` protocol (written as `C1.Item: Equatable`).

The first and second requirements are defined in the function’s type parameter list, and the third and fourth requirements are defined in the function’s generic `where` clause.

These requirements mean:

• `someContainer` is a container of type `C1`.
• `anotherContainer` is a container of type `C2`.
• `someContainer` and `anotherContainer` contain the same type of items.
• The items in `someContainer` can be checked with the not equal operator (`!=`) to see if they’re different from each other.

The third and fourth requirements combine to mean that the items in `anotherContainer` can also be checked with the `!=` operator, because they’re exactly the same type as the items in `someContainer`.

These requirements enable the `allItemsMatch(_:_:)` function to compare the two containers, even if they’re of a different container type.

The `allItemsMatch(_:_:)` function starts by checking that both containers contain the same number of items. If they contain a different number of items, there’s no way that they can match, and the function returns `false`.

After making this check, the function iterates over all of the items in `someContainer` with a `for``in` loop and the half-open range operator (`..<`). For each item, the function checks whether the item from `someContainer` isn’t equal to the corresponding item in `anotherContainer`. If the two items aren’t equal, then the two containers don’t match, and the function returns `false`.

If the loop finishes without finding a mismatch, the two containers match, and the function returns `true`.

Here’s how the `allItemsMatch(_:_:)` function looks in action:

var stackOfStrings = Stack<String>()
stackOfStrings.push(“uno”)
stackOfStrings.push(“dos”)
stackOfStrings.push(“tres”)
var arrayOfStrings = [“uno”, “dos”, “tres”]
if allItemsMatch(stackOfStrings, arrayOfStrings) {
print(“All items match.”)
} else {
print(“Not all items match.”)
}
// Prints “All items match.”

The example above creates a `Stack` instance to store `String` values, and pushes three strings onto the stack. The example also creates an `Array` instance initialized with an array literal containing the same three strings as the stack. Even though the stack and the array are of a different type, they both conform to the `Container` protocol, and both contain the same type of values. You can therefore call the `allItemsMatch(_:_:)`function with these two containers as its arguments. In the example above, the `allItemsMatch(_:_:)` function correctly reports that all of the items in the two containers match.

## Extensions with a Generic Where Clause

You can also use a generic `where` clause as part of an extension. The example below extends the generic `Stack` structure from the previous examples to add an `isTop(_:)` method.

// Element 는

Equatable protocol을 conform해야만 한다.

extension Stack where Element: Equatable {
func isTop(_ item: Element) -> Bool {

// 맨위 아이템이 있는지 확인
guard let topItem = items.last else {
return false
}
}
}

This new `isTop(_:)` method first checks that the stack isn’t empty, and then compares the given item against the stack’s topmost item. If you tried to do this without a generic `where` clause, you would have a problem: The implementation of `isTop(_:)` uses the `==` operator, but the definition of `Stack` doesn’t require its items to be equatable, so using the `==` operator results in a compile-time error. Using a generic `where` clause lets you add a new requirement to the extension, so that the extension adds the `isTop(_:)` method only when the items in the stack are equatable.

Here’s how the `isTop(_:)` method looks in action:

if stackOfStrings.isTop(“tres”) {
print(“Top element is tres.”)
} else {
print(“Top element is something else.”)
}
// Prints “Top element is tres.”

If you try to call the `isTop(_:)` method on a stack whose elements aren’t equatable, you’ll get a compile-time error.

struct NotEquatable { }
var notEquatableStack = Stack<NotEquatable>()

// 임의로

Equatable 를 conform 하지않는 요소 만들기
let notEquatableValue = NotEquatable()
notEquatableStack.push(notEquatableValue)
notEquatableStack.isTop(notEquatableValue)  // Error

You can use a generic `where` clause with extensions to a protocol. The example below extends the `Container`protocol from the previous examples to add a `startsWith(_:)` method.

extension Container where Item: Equatable {
func startsWith(_ item: Item) -> Bool {
return count >= 1 && self[0] == item
}
}

The `startsWith(_:)` method first makes sure that the container has at least one item, and then it checks whether the first item in the container matches the given item. This new `startsWith(_:)` method can be used with any type that conforms to the `Container` protocol, including the stacks and arrays used above, as long as the container’s items are equatable.

if [9, 9, 9].startsWith(42) {
print(“Starts with 42.”)
} else {
print(“Starts with something else.”)
}
// Prints “Starts with something else.”

The generic `where` clause in the example above requires `Item` to conform to a protocol, but you can also write a generic `where` clauses that require `Item` to be a specific type. For example:

extension Container where Item == Double {
func average() -> Double {
var sum = 0.0
for index in 0..<count {
sum += self[index]
}
return sum / Double(count)
}
}
print([1260.0, 1200.0, 98.6, 37.0].average())
// Prints “648.9”

This example adds an `average()` method to containers whose `Item` type is `Double`. It iterates over the items in the container to add them up, and divides by the container’s count to compute the average. It explicitly converts the count from `Int` to `Double` to be able to do floating-point division.

You can include multiple requirements in a generic `where` clause that is part of an extension, just like you can for a generic `where` clause that you write elsewhere. Separate each requirement in the list with a comma.

## Associated Types with a Generic Where Clause

You can include a generic `where` clause on an associated type. For example, suppose you want to make a version of `Container` that includes an iterator, like what the `Sequence` protocol uses in the standard library. Here’s how you write that:

protocol Container {
associatedtype Item
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }

associatedtype Iterator: IteratorProtocol where Iterator.Element == Item
func makeIterator() -> Iterator
}

The generic `where` clause on `Iterator` requires that the iterator must traverse over elements of the same item type as the container’s items, regardless of the iterator’s type. The `makeIterator()` function provides access to a container’s iterator.

For a protocol that inherits from another protocol, you add a constraint to an inherited associated type by including the generic `where` clause in the protocol declaration. For example, the following code declares a `ComparableContainer` protocol that requires `Item` to conform to `Comparable`:

protocol ComparableContainer: Container where Item: Comparable { }

## Generic Subscripts

Subscripts can be generic, and they can include generic `where` clauses. You write the placeholder type name inside angle brackets after `subscript`, and you write a generic `where` clause right before the opening curly brace of the subscript’s body. For example:

extension Container {
subscript<Indices: Sequence>(indices: Indices) -> [Item]
where Indices.Iterator.Element == Int {
var result = [Item]()
for index in indices {
result.append(self[index])
}
return result
}
}

This extension to the `Container` protocol adds a subscript that takes a sequence of indices and returns an array containing the items at each given index. This generic subscript is constrained as follows:

The generic parameter `Indices` in angle brackets has to be a type that conforms to the `Sequence`protocol from the standard library.

The subscript takes a single parameter, `indices`, which is an instance of that `Indices` type.

The generic `where` clause requires that the iterator for the sequence must traverse over elements of type `Int`. This ensures that the indices in the sequence are the same type as the indices used for a container.

Taken together, these constraints mean that the value passed for the `indices` parameter is a sequence of integers.

## Generics

For example, Swift’s `Array` and `Dictionary` types are both generic collections. You can create an array that holds `Int` values, or an array that holds `String` values, or indeed an array for any other type that can be created in Swift. Similarly, you can create a dictionary to store values of any specified type, and there are no limitations on what that type can be.

## The Problem That Generics Solve

Here’s a standard, nongeneric function called `swapTwoInts(_:_:)`, which swaps two `Int` values:

func swapTwoInts(_ a: inout Int, _ b: inout Int) {
let temporaryA = a
a = b
b = temporaryA
}

var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt, &anotherInt)
print(“someInt is now (someInt), and anotherInt is now (anotherInt)”)
// Prints “someInt is now 107, and anotherInt is now 3”

The `swapTwoInts(_:_:)` function is useful, but it can only be used with `Int` values. If you want to swap two `String` values, or two `Double` values, you have to write more functions, such as the `swapTwoStrings(_:_:)` and `swapTwoDoubles(_:_:)` functions shown below:

func swapTwoStrings(_ a: inout String, _ b: inout String) {
let temporaryA = a
a = b
b = temporaryA
}

func swapTwoDoubles(_ a: inout Double, _ b: inout Double) {
let temporaryA = a
a = b
b = temporaryA
}

It’s more useful, and considerably more flexible, to write a single function that swaps two values of any type. Generic code enables you to write such a function. (A generic version of these functions is defined below.)

## Generic Functions

func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
let temporaryA = a
a = b
b = temporaryA
}

1. `func swapTwoInts(_ a: inout Int, _ b: inout Int)`
2. `func swapTwoValues<T>(_ a: inout T, _ b: inout T)`

The generic version of the function uses a placeholder type name (called `T`, in this case) instead of an actual type name (such as `Int`, `String`, or `Double`). The placeholder type name doesn’t say anything about what `T `must be, but it does say that both `a` and `b` must be of the same type `T`, whatever `T` represents. The actual type to use in place of `T` is determined each time the `swapTwoValues(_:_:)` function is called.

var someInt = 3

var anotherInt = 107
swapTwoValues(&someInt, &anotherInt)
// someInt is now 107, and anotherInt is now 3
var someString = “hello”
var anotherString = “world”
swapTwoValues(&someString, &anotherString)
// someString is now “world”, and anotherString is now “hello”

## Type Parameters

In the `swapTwoValues(_:_:)` example above, the placeholder type `T` is an example of a type parameter. Type parameters specify and name a placeholder type, and are written immediately after the function’s name, between a pair of matching angle brackets (such as `<T>`).

Once you specify a type parameter, you can use it to define the type of a function’s parameters (such as the `a` and `b` parameters of the `swapTwoValues(_:_:)` function), or as the function’s return type, or as a type annotation within the body of the function.

You can provide more than one type parameter by writing multiple type parameter names within the angle brackets, separated by commas.

## Naming Type Parameters

In most cases, type parameters have descriptive names, such as `Key` and `Value` in `Dictionary<Key, Value>`and `Element` in `Array<Element>`, which tells the reader about the relationship between the type parameter and the generic type or function it’s used in. However, when there isn’t a meaningful relationship between them, it’s traditional to name them using single letters such as `T`, `U`, and `V`.

NOTE

Always give type parameters upper camel case names (such as `T` and `MyTypeParameter`

## Generic Types

In addition to generic functions, Swift enables you to define your own generic types. These are custom classes, structures, and enumerations that can work with any type, in a similar way to `Array` and `Dictionary`.

The illustration below shows the push and pop behavior for a stack:

1. There are currently three values on the stack.
2. A fourth value is pushed onto the top of the stack.
3. The stack now holds four values, with the most recent one at the top.
4. The top item in the stack is popped.
5. After popping a value, the stack once again holds three values.

Here’s how to write a nongeneric version of a stack, in this case for a stack of `Int` values:

struct IntStack {
var items = [Int]()
mutating func push(_ item: Int) {
items.append(item)
}
mutating func pop() -> Int {
return items.removeLast()
}
}

This structure uses an `Array` property called `items` to store the values in the stack. `Stack` provides two methods, `push` and `pop`, to push and pop values on and off the stack. These methods are marked as `mutating`, because they need to modify (or mutate) the structure’s `items` array.

The `IntStack` type shown above can only be used with `Int` values, however. It would be much more useful to define a generic `Stack` class, that can manage a stack of any type of value.

Here’s a generic version of the same code:

struct Stack<Element> {
var items = [Element]()
mutating func push(_ item: Element) {
items.append(item)
}
mutating func pop() -> Element {
return items.removeLast()
}
}

var stackOfStrings = Stack<String>()
stackOfStrings.push(“uno”)
stackOfStrings.push(“dos”)
stackOfStrings.push(“tres”)
stackOfStrings.push(“cuatro”)
// the stack now contains 4 strings

let fromTheTop = stackOfStrings.pop()
// fromTheTop is equal to “cuatro”, and the stack now contains 3 strings

## Extending a Generic Type

When you extend a generic type, you don’t provide a type parameter list as part of the extension’s definition. Instead, the type parameter list from the original type definition is available within the body of the extension, and the original type parameter names are used to refer to the type parameters from the original definition.

(예를 들어 아래 예시에서 Element가 type parameter 이며 이것이 extending하는 부분에서 접근가능하다는 이야기)

struct Stack<Element> {
}

extension Stack {

// Element가 extended 정의의 type parameter이므로 접근 가능
var topItem: Element? {
return items.isEmpty ? nil : items[items.count – 1]
}
}

if let topItem = stackOfStrings.topItem {
print(“The top item on the stack is (topItem).”)
}
// Prints “The top item on the stack is tres.”

## Type Constraints

Type constraints specify that a type parameter must inherit from a specific class, or conform to a particular protocol or protocol composition.

For example, Swift’s `Dictionary` type places a limitation on the types that can be used as keys for a dictionary. As described in Dictionaries, the type of a dictionary’s keys must be hashable. That is, it must provide a way to make itself uniquely representable. `Dictionary` needs its keys to be hashable so that it can check whether it already contains a value for a particular key. Without this requirement, `Dictionary` could not tell whether it should insert or replace a value for a particular key, nor would it be able to find a value for a given key that is already in the dictionary.

This requirement is enforced by a type constraint on the key type for `Dictionary`, which specifies that the key type must conform to the `Hashable` protocol, a special protocol defined in the Swift standard library. All of Swift’s basic types (such as `String`, `Int`, `Double`, and `Bool`) are hashable by default.

You can define your own type constraints when creating custom generic types, and these constraints provide much of the power of generic programming. Abstract concepts like `Hashable` characterize types in terms of their conceptual characteristics, rather than their concrete type.

## Type Constraint Syntax

You write type constraints by placing a single class or protocol constraint after a type parameter’s name, separated by a colon, as part of the type parameter list. The basic syntax for type constraints on a generic function is shown below (although the syntax is the same for generic types):

func someFunction<T: SomeClass, U: SomeProtocol>(someT: T, someU: U) {
// function body goes here
}

The hypothetical function above has two type parameters. The first type parameter, `T`, has a type constraint that requires `T` to be a subclass of `SomeClass`. The second type parameter, `U`, has a type constraint that requires `U` to conform to the protocol `SomeProtocol`.

## Type Constraints in Action

type constaints의 사용 예시

Here’s a nongeneric function called `findIndex(ofString:in:)`, which is given a `String` value to find and an array of `String` values within which to find it. The `findIndex(ofString:in:)` function returns an optional `Int`value, which will be the index of the first matching string in the array if it’s found, or `nil` if the string can’t be found:

func findIndex(ofString valueToFind: String, in array: [String]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}

The `findIndex(ofString:in:)` function can be used to find a string value in an array of strings:

let strings = [“cat”, “dog”, “llama”, “parakeet”, “terrapin”]
if let foundIndex = findIndex(ofString: “llama”, in: strings) {
print(“The index of llama is (foundIndex)”)
}
// Prints “The index of llama is 2”

The principle of finding the index of a value in an array isn’t useful only for strings, however. You can write the same functionality as a generic function by replacing any mention of strings with values of some type `T`instead.

Here’s how you might expect a generic version of `findIndex(ofString:in:)`, called `findIndex(of:in:)`, to be written. Note that the return type of this function is still `Int?`, because the function returns an optional index number, not an optional value from the array. Be warned, though—this function doesn’t compile, for reasons explained after the example:

func findIndex<T>(of valueToFind: T, in array:[T]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}

This function doesn’t compile as written above. The problem lies with the equality check, “`if value == valueToFind`”. Not every type in Swift can be compared with the equal to operator (`==`). If you create your own class or structure to represent a complex data model, for example, then the meaning of “equal to” for that class or structure isn’t something that Swift can guess for you. Because of this, it isn’t possible to guarantee that this code will work for every possible type `T`, and an appropriate error is reported when you try to compile the code.

All is not lost, however. The Swift standard library defines a protocol called `Equatable`, which requires any conforming type to implement the equal to operator (`==`) and the not equal to operator (`!=`) to compare any two values of that type. All of Swift’s standard types automatically support the `Equatable` protocol.

Any type that is `Equatable` can be used safely with the `findIndex(of:in:)` function, because it’s guaranteed to support the equal to operator. To express this fact, you write a type constraint of `Equatable` as part of the type parameter’s definition when you define the function:

func findIndex<T: Equatable>(of valueToFind: T, in array:[T]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}

The single type parameter for `findIndex(of:in:)` is written as `T: Equatable`, which means “any type `T` that conforms to the `Equatable` protocol.”

The `findIndex(of:in:)` function now compiles successfully and can be used with any type that is `Equatable`, such as `Double` or `String`:

let doubleIndex = findIndex(of: 9.3, in: [3.14159, 0.1, 0.25])
// doubleIndex is an optional Int with no value, because 9.3 isn’t in the array
let stringIndex = findIndex(of: “Andrea”, in: [“Mike”, “Malcolm”, “Andrea”])
// stringIndex is an optional Int containing a value of 2

## Associated Types

참고 사항) protocol에서 generic을 사용해야 하는 경우는 위에서 사용한 방법들과는 다르다. <T>의 방법은 사용 불가능하고 associatedtype을 대신 사용한다. 그 이전에는 typealias를 이용했으나 deprecated되었다.

참고 사항)

When defining a protocol, it’s sometimes useful to declare one or more associated types as part of the protocol’s definition. An associated type gives a placeholder name to a type that is used as part of the protocol. The actual type to use for that associated type isn’t specified until the protocol is adopted. Associated types are specified with the `associatedtype` keyword.

Associated Types in Action

Here’s an example of a protocol called `Container`, which declares an associated type called `Item`:

protocol Container {
associatedtype Item
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }
}

The `Container` protocol defines three required capabilities that any container must provide:

• It must be possible to add a new item to the container with an `append(_:)` method.
• It must be possible to access a count of the items in the container through a `count` property that returns an `Int` value.
• It must be possible to retrieve each item in the container with a subscript that takes an `Int` index value.

This protocol doesn’t specify how the items in the container should be stored or what type they’re allowed to be. The protocol only specifies the three bits of functionality that any type must provide in order to be considered a `Container`. A conforming type can provide additional functionality, as long as it satisfies these three requirements.

Any type that conforms to the `Container` protocol must be able to specify the type of values it stores. Specifically, it must ensure that only items of the right type are added to the container, and it must be clear about the type of the items returned by its subscript.

To define these requirements, the `Container` protocol needs a way to refer to the type of the elements that a container will hold, without knowing what that type is for a specific container. The `Container` protocol needs to specify that any value passed to the `append(_:)` method must have the same type as the container’s element type, and that the value returned by the container’s subscript will be of the same type as the container’s element type.

To achieve this, the `Container` protocol declares an associated type called `Item`, written as `associatedtype Item`. The protocol doesn’t define what `Item` is—that information is left for any conforming type to provide.(generic type은 protocol 에서 정의되는 것이 아니고 conform하는 부분에서 수행된다.) Nonetheless, the `Item` alias provides a way to refer to the type of the items in a `Container`, and to define a type for use with the `append(_:)` method and subscript, to ensure that the expected behavior of any `Container` is enforced.

Here’s a version of the nongeneric `IntStack` type from earlier, adapted to conform to the `Container` protocol:

struct IntStack: Container {
// original IntStack implementation
var items = [Int]()
mutating func push(_ item: Int) {
items.append(item)
}
mutating func pop() -> Int {
return items.removeLast()
}

// conformance to the Container protocol

// 아래에서 설명하겠지만 이렇게 명시적으로 하는 방법이 있지만 암묵적으로도가능하다.
typealias Item = Int

mutating func append(_ item: Int) {
self.push(item)
}
var count: Int {
return items.count
}
subscript(i: Int) -> Int {
return items[i]
}
}

The `IntStack` type implements all three of the `Container` protocol’s requirements, and in each case wraps part of the `IntStack` type’s existing functionality to satisfy these requirements.

Moreover, `IntStack` specifies that for this implementation of `Container`, the appropriate `Item` to use is a type of `Int`. The definition of `typealias Item = Int` turns the abstract type of `Item` into a concrete type of `Int` for this implementation of the `Container` protocol.

Thanks to Swift’s type inference, you don’t actually need to declare a concrete `Item` of `Int` as part of the definition of `IntStack`. Because `IntStack` conforms to all of the requirements of the `Container` protocol, Swift can infer the appropriate `Item` to use, simply by looking at the type of the `append(_:)` method’s `item`parameter and the return type of the subscript. Indeed, if you delete the `typealias Item = Int` line from the code above, everything still works, because it’s clear what type should be used for `Item`. (명시적으로    typealias Item = Int 이렇게 하지 않아도 암묵적으로 swift가 알맞은 type을 찾을수 있다.)

You can also make the generic `Stack` type conform to the `Container` protocol:

struct Stack<Element>: Container {
// original Stack<Element> implementation
var items = [Element]()
mutating func push(_ item: Element) {
items.append(item)
}
mutating func pop() -> Element {
return items.removeLast()
}
// conformance to the Container protocol
mutating func append(_ item: Element) {
self.push(item)
}
var count: Int {
return items.count
}
subscript(i: Int) -> Element {
return items[i]
}
}

This time, the type parameter `Element` is used as the type of the `append(_:)` method’s `item` parameter and the return type of the subscript. Swift can therefore infer that `Element` is the appropriate type to use as the `Item `for this particular container.

Extending an Existing Type to Specify an Associated Type

You can extend an existing type to add conformance to a protocol, as described in Adding Protocol Conformance with an Extension. This includes a protocol with an associated type.

Swift’s `Array` type already provides an `append(_:)` method, a `count` property, and a subscript with an `Int `index to retrieve its elements. These three capabilities match the requirements of the `Container` protocol. This means that you can extend `Array` to conform to the `Container` protocol simply by declaring that `Array `adopts the protocol. You do this with an empty extension, as described in Declaring Protocol Adoption with an Extension:

extension Array: Container {}

Array’s existing `append(_:)` method and subscript enable Swift to infer the appropriate type to use for `Item`, just as for the generic `Stack` type above. After defining this extension, you can use any `Array` as a `Container`.

Using Type Annotations to Constrain an Associated Type

You can add a type annotation to an associated type in a protocol, to require that conforming types satisfy the constraints described by the type annotation. For example, the following code defines a version of `Container` that requires the items in the container to be equatable.

protocol Container {

// Item 은 Equatable protocol을 conform하는 것이어야 한다.
associatedtype Item: Equatable
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }
}

To conform to this version of `Container`, the container’s `Item` type has to conform to the `Equatable` protocol.

Using Type Annotations to Constrain an Associated Type

You can add a type annotation to an associated type in a protocol, to require that conforming types satisfy the constraints described by the type annotation. For example, the following code defines a version of `Container` that requires the items in the container to be equatable.

protocol Container {
associatedtype Item: Equatable
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }
}

To conform to this version of `Container`, the container’s `Item` type has to conform to the `Equatable` protocol.

Using a Protocol in Its Associated Type’s Constraints

## (이해 못했던 부분)

A protocol can appear as part of its own requirements. For example, here’s a protocol that refines the `Container` protocol, adding the requirement of a `suffix(_:)` method. The `suffix(_:)` method returns a given number of elements from the end of the container, storing them in an instance of the `Suffix` type.

protocol SuffixableContainer: Container {
associatedtype Suffix: SuffixableContainer where Suffix.Item == Item
func suffix(_ size: Int) -> Suffix
}

In this protocol, `Suffix` is an associated type, like the `Item` type in the `Container` example above. `Suffix` has two constraints: It must conform to the `SuffixableContainer` protocol (the protocol currently being defined), and its `Item` type must be the same as the container’s `Item` type. The constraint on `Item` is a generic `where`clause, which is discussed in Associated Types with a Generic Where Clause below.

Here’s an extension of the `Stack` type from earlier that adds conformance to the `SuffixableContainer`protocol:

extension Stack: SuffixableContainer {
func suffix(_ size: Int) -> Stack {
var result = Stack()
for index in (count-size)..<count {
result.append(self[index])
}
return result
}
// Inferred that Suffix is Stack.
}
var stackOfInts = Stack<Int>()
stackOfInts.append(10)
stackOfInts.append(20)
stackOfInts.append(30)
let suffix = stackOfInts.suffix(2)
// suffix contains 20 and 30

In the example above, the `Suffix` associated type for `Stack` is also `Stack`, so the suffix operation on `Stack`returns another `Stack`. Alternatively, a type that conforms to `SuffixableContainer` can have a `Suffix` type that’s different from itself—meaning the suffix operation can return a different type. For example, here’s an extension to the nongeneric `IntStack` type that adds `SuffixableContainer` conformance, using `Stack<Int>` as its suffix type instead of `IntStack`:

extension IntStack: SuffixableContainer {
func suffix(_ size: Int) -> Stack<Int> {
var result = Stack<Int>()
for index in (count-size)..<count {
result.append(self[index])
}
return result
}
// Inferred that Suffix is Stack<Int>.
}

## Generic Where Clauses

Type constraints, as described in Type Constraints, enable you to define requirements on the type parameters associated with a generic function, subscript, or type.

It can also be useful to define requirements for associated types. You do this by defining a generic where clause. A generic `where` clause enables you to require that an associated type must conform to a certain protocol, or that certain type parameters and associated types must be the same. A generic `where` clause starts with the `where` keyword, followed by constraints for associated types or equality relationships between types and associated types. You write a generic `where` clause right before the opening curly brace of a type or function’s body.

The example below defines a generic function called `allItemsMatch`, which checks to see if two `Container`instances contain the same items in the same order. The function returns a Boolean value of `true` if all items match and a value of `false` if they don’t.

The two containers to be checked don’t have to be the same type of container (although they can be), but they do have to hold the same type of items. This requirement is expressed through a combination of type constraints and a generic `where` clause:

//

<C1: Container, C2: Container> 이부분

//

defining a generic where clause

func allItemsMatch<C1: Container, C2: Container>
(_ someContainer: C1, _ anotherContainer: C2) -> Bool
where C1.Item == C2.Item, C1.Item: Equatable {

// Check that both containers contain the same number of items.
if someContainer.count != anotherContainer.count {
return false
}

// Check each pair of items to see if they’re equivalent.
for i in 0..<someContainer.count {
if someContainer[i] != anotherContainer[i] {
return false
}
}

// All items match, so return true.
return true
}

This function takes two arguments called `someContainer` and `anotherContainer`. The `someContainer` argument is of type `C1`, and the `anotherContainer` argument is of type `C2`. Both `C1` and `C2` are type parameters for two container types to be determined when the function is called.

The following requirements are placed on the function’s two type parameters:

• `C1` must conform to the `Container` protocol (written as `C1: Container`).
• `C2` must also conform to the `Container` protocol (written as `C2: Container`).
• The `Item` for `C1` must be the same as the `Item` for `C2` (written as `C1.Item == C2.Item`).
• The `Item` for `C1` must conform to the `Equatable` protocol (written as `C1.Item: Equatable`).

The first and second requirements are defined in the function’s type parameter list, and the third and fourth requirements are defined in the function’s generic `where` clause.

These requirements mean:

• `someContainer` is a container of type `C1`.
• `anotherContainer` is a container of type `C2`.
• `someContainer` and `anotherContainer` contain the same type of items.
• The items in `someContainer` can be checked with the not equal operator (`!=`) to see if they’re different from each other.

The third and fourth requirements combine to mean that the items in `anotherContainer` can also be checked with the `!=` operator, because they’re exactly the same type as the items in `someContainer`.

These requirements enable the `allItemsMatch(_:_:)` function to compare the two containers, even if they’re of a different container type.

The `allItemsMatch(_:_:)` function starts by checking that both containers contain the same number of items. If they contain a different number of items, there’s no way that they can match, and the function returns `false`.

After making this check, the function iterates over all of the items in `someContainer` with a `for``in` loop and the half-open range operator (`..<`). For each item, the function checks whether the item from `someContainer` isn’t equal to the corresponding item in `anotherContainer`. If the two items aren’t equal, then the two containers don’t match, and the function returns `false`.

If the loop finishes without finding a mismatch, the two containers match, and the function returns `true`.

Here’s how the `allItemsMatch(_:_:)` function looks in action:

var stackOfStrings = Stack<String>()
stackOfStrings.push(“uno”)
stackOfStrings.push(“dos”)
stackOfStrings.push(“tres”)
var arrayOfStrings = [“uno”, “dos”, “tres”]
if allItemsMatch(stackOfStrings, arrayOfStrings) {
print(“All items match.”)
} else {
print(“Not all items match.”)
}
// Prints “All items match.”

The example above creates a `Stack` instance to store `String` values, and pushes three strings onto the stack. The example also creates an `Array` instance initialized with an array literal containing the same three strings as the stack. Even though the stack and the array are of a different type, they both conform to the `Container` protocol, and both contain the same type of values. You can therefore call the `allItemsMatch(_:_:)`function with these two containers as its arguments. In the example above, the `allItemsMatch(_:_:)` function correctly reports that all of the items in the two containers match.

## Extensions with a Generic Where Clause

You can also use a generic `where` clause as part of an extension. The example below extends the generic `Stack` structure from the previous examples to add an `isTop(_:)` method.

// Element 는

Equatable protocol을 conform해야만 한다.

extension Stack where Element: Equatable {
func isTop(_ item: Element) -> Bool {

// 맨위 아이템이 있는지 확인
guard let topItem = items.last else {
return false
}
}
}

This new `isTop(_:)` method first checks that the stack isn’t empty, and then compares the given item against the stack’s topmost item. If you tried to do this without a generic `where` clause, you would have a problem: The implementation of `isTop(_:)` uses the `==` operator, but the definition of `Stack` doesn’t require its items to be equatable, so using the `==` operator results in a compile-time error. Using a generic `where` clause lets you add a new requirement to the extension, so that the extension adds the `isTop(_:)` method only when the items in the stack are equatable.

Here’s how the `isTop(_:)` method looks in action:

if stackOfStrings.isTop(“tres”) {
print(“Top element is tres.”)
} else {
print(“Top element is something else.”)
}
// Prints “Top element is tres.”

If you try to call the `isTop(_:)` method on a stack whose elements aren’t equatable, you’ll get a compile-time error.

struct NotEquatable { }
var notEquatableStack = Stack<NotEquatable>()

// 임의로

Equatable 를 conform 하지않는 요소 만들기
let notEquatableValue = NotEquatable()
notEquatableStack.push(notEquatableValue)
notEquatableStack.isTop(notEquatableValue)  // Error

You can use a generic `where` clause with extensions to a protocol. The example below extends the `Container`protocol from the previous examples to add a `startsWith(_:)` method.

extension Container where Item: Equatable {
func startsWith(_ item: Item) -> Bool {
return count >= 1 && self[0] == item
}
}

The `startsWith(_:)` method first makes sure that the container has at least one item, and then it checks whether the first item in the container matches the given item. This new `startsWith(_:)` method can be used with any type that conforms to the `Container` protocol, including the stacks and arrays used above, as long as the container’s items are equatable.

if [9, 9, 9].startsWith(42) {
print(“Starts with 42.”)
} else {
print(“Starts with something else.”)
}
// Prints “Starts with something else.”

The generic `where` clause in the example above requires `Item` to conform to a protocol, but you can also write a generic `where` clauses that require `Item` to be a specific type. For example:

extension Container where Item == Double {
func average() -> Double {
var sum = 0.0
for index in 0..<count {
sum += self[index]
}
return sum / Double(count)
}
}
print([1260.0, 1200.0, 98.6, 37.0].average())
// Prints “648.9”

This example adds an `average()` method to containers whose `Item` type is `Double`. It iterates over the items in the container to add them up, and divides by the container’s count to compute the average. It explicitly converts the count from `Int` to `Double` to be able to do floating-point division.

You can include multiple requirements in a generic `where` clause that is part of an extension, just like you can for a generic `where` clause that you write elsewhere. Separate each requirement in the list with a comma.

## Associated Types with a Generic Where Clause

You can include a generic `where` clause on an associated type. For example, suppose you want to make a version of `Container` that includes an iterator, like what the `Sequence` protocol uses in the standard library. Here’s how you write that:

protocol Container {
associatedtype Item
mutating func append(_ item: Item)
var count: Int { get }
subscript(i: Int) -> Item { get }

associatedtype Iterator: IteratorProtocol where Iterator.Element == Item
func makeIterator() -> Iterator
}

The generic `where` clause on `Iterator` requires that the iterator must traverse over elements of the same item type as the container’s items, regardless of the iterator’s type. The `makeIterator()` function provides access to a container’s iterator.

For a protocol that inherits from another protocol, you add a constraint to an inherited associated type by including the generic `where` clause in the protocol declaration. For example, the following code declares a `ComparableContainer` protocol that requires `Item` to conform to `Comparable`:

protocol ComparableContainer: Container where Item: Comparable { }

## Generic Subscripts

Subscripts can be generic, and they can include generic `where` clauses. You write the placeholder type name inside angle brackets after `subscript`, and you write a generic `where` clause right before the opening curly brace of the subscript’s body. For example:

extension Container {
subscript<Indices: Sequence>(indices: Indices) -> [Item]
where Indices.Iterator.Element == Int {
var result = [Item]()
for index in indices {
result.append(self[index])
}
return result
}
}

This extension to the `Container` protocol adds a subscript that takes a sequence of indices and returns an array containing the items at each given index. This generic subscript is constrained as follows:

The generic parameter `Indices` in angle brackets has to be a type that conforms to the `Sequence`protocol from the standard library.

The subscript takes a single parameter, `indices`, which is an instance of that `Indices` type.

The generic `where` clause requires that the iterator for the sequence must traverse over elements of type `Int`. This ensures that the indices in the sequence are the same type as the indices used for a container.

Taken together, these constraints mean that the value passed for the `indices` parameter is a sequence of integers.

## Extensions

Extensions add new functionality to an existing class, structure, enumeration, or protocol type. This includes the ability to extend types for which you do not have access to the original source code (known as retroactive modeling).

Extensions in Swift can:

• Add computed instance properties and computed type properties (stored property는 추가 할수 없다.)
• Define instance methods and type methods
• Provide new initializers
• Define subscripts
• Define and use new nested types
• Make an existing type conform to a protocol

In Swift, you can even extend a protocol to provide implementations of its requirements or add additional functionality that conforming types can take advantage of. For more details, see Protocol Extensions.

NOTE

Extensions can add new functionality to a type, but they cannot override existing functionality.

## Extension Syntax

Declare extensions with the `extension` keyword:

extension SomeType {
// new functionality to add to SomeType goes here
}

An extension can extend an existing type to make it adopt one or more protocols. To add protocol conformance, you write the protocol names the same way as you write them for a class or structure:

extension SomeType: SomeProtocol, AnotherProtocol {
// implementation of protocol requirements goes here
}

Adding protocol conformance in this way is described in Adding Protocol Conformance with an Extension.

An extension can be used to extend an existing generic type, as described in Extending a Generic Type. You can also extend a generic type to conditionally add functionality, as described in Extensions with a Generic Where Clause.

NOTE

If you define an extension to add new functionality to an existing type, the new functionality will be available on all existing instances of that type, even if they were created before the extension was defined

## Computed Properties

Extensions can add computed instance properties and computed type properties to existing types. This example adds five computed instance properties to Swift’s built-in `Double` type, to provide basic support for working with distance units:

extension Double {
var km: Double { return self * 1_000.0 }
var m: Double { return self }
var cm: Double { return self / 100.0 }
var mm: Double { return self / 1_000.0 }
var ft: Double { return self / 3.28084 }
}
let oneInch = 25.4.mm
print(“One inch is (oneInch) meters”)
// Prints “One inch is 0.0254 meters”
let threeFeet = 3.ft
print(“Three feet is (threeFeet) meters”)
// Prints “Three feet is 0.914399970739201 meters”

let aMarathon = 42.km + 195.m
print(“A marathon is (aMarathon) meters long”)
// Prints “A marathon is 42195.0 meters long”

NOTE

Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.

## Initializers

Extensions can add new initializers to existing types.

Extensions can add new convenience initializers to a class, but they cannot add new designated initializers or deinitializers to a class. Designated initializers and deinitializers must always be provided by the original class implementation.

NOTE

If you use an extension to add an initializer to a value type that provides default values for all of its stored properties and does not define any custom initializers, you can call the default initializer and memberwise initializer for that value type from within your extension’s initializer.

This would not be the case if you had written the initializer as part of the value type’s original implementation, as described in Initializer Delegation for Value Types.

struct Size {
var width = 0.0, height = 0.0
}
struct Point {
var x = 0.0, y = 0.0
}
struct Rect {
var origin = Point()
var size = Size()
}

let defaultRect = Rect()
let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0),
size: Size(width: 5.0, height: 5.0))

You can extend the `Rect` structure to provide an additional initializer that takes a specific center point and size:

extension Rect {
init(center: Point, size: Size) {
let originX = center.x – (size.width / 2)
let originY = center.y – (size.height / 2)
self.init(origin: Point(x: originX, y: originY), size: size)
}
}

This new initializer starts by calculating an appropriate origin point based on the provided `center` point and `size` value. The initializer then calls the structure’s automatic memberwise initializer `init(origin:size:)`, which stores the new origin and size values in the appropriate properties:

let centerRect = Rect(center: Point(x: 4.0, y: 4.0),
size: Size(width: 3.0, height: 3.0))
// centerRect’s origin is (2.5, 2.5) and its size is (3.0, 3.0)

## Methods

Extensions can add new instance methods and type methods to existing types. The following example adds a new instance method called `repetitions` to the `Int` type:

extension Int {
func repetitions(task: () -> Void) {

// 0과 self사이의 정수를 인덱스로 순환

for _ in 0..<self {
}
}
}

3.repetitions {
print(“Hello!”)
}
// Hello!
// Hello!
// Hello!

Mutating Instance Methods

(참고사항 tumblr #swift #mutating #enum:

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.)

Instance methods added with an extension can also modify (or mutate) the instance itself. Structure and enumeration methods that modify `self` or its properties must mark the instance method as `mutating`, just like mutating methods from an original implementation.

extension Int {
mutating func square() {
self = self * self
}
}
var someInt = 3
someInt.square()
// someInt is now 9

## Subscripts

Extensions can add new subscripts to an existing type.

`123456789[0]` returns `9`

`123456789[1]` returns `8`

extension Int {
subscript(digitIndex: Int) -> Int {
var decimalBase = 1
for _ in 0..<digitIndex {
decimalBase *= 10
}
return (self / decimalBase) % 10
}
}
746381295[0]
// returns 5
746381295[1]
// returns 9
746381295[2]
// returns 2
746381295[8]
// returns 7

746381295[9]
// returns 0, as if you had requested:
0746381295[9]

## Nested Types

Extensions can add new nested types to existing classes, structures, and enumerations:

extension Int {
enum Kind {
case negative, zero, positive
}
var kind: Kind {
switch self {
case 0:
return .zero
case let x where x > 0:
return .positive
default:
return .negative
}
}
}

func printIntegerKinds(_ numbers: [Int]) {
for number in numbers {
switch number.kind {
case .negative:
print(“- ”, terminator: “”)
case .zero:
print(“0 ”, terminator: “”)
case .positive:
print(“+ ”, terminator: “”)
}
}
print(“”)
}
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])
// Prints “+ + – 0 – 0 + ”

NOTE

`number.kind` is already known to be of type `Int.Kind`. Because of this, all of the `Int.Kind` case values can be written in shorthand form inside the `switch` statement, such as `.negative` rather than `Int.Kind.negative`

## Extensions

Extensions add new functionality to an existing class, structure, enumeration, or protocol type. This includes the ability to extend types for which you do not have access to the original source code (known as retroactive modeling).

Extensions in Swift can:

• Add computed instance properties and computed type properties (stored property는 추가 할수 없다.)
• Define instance methods and type methods
• Provide new initializers
• Define subscripts
• Define and use new nested types
• Make an existing type conform to a protocol

In Swift, you can even extend a protocol to provide implementations of its requirements or add additional functionality that conforming types can take advantage of. For more details, see Protocol Extensions.

NOTE

Extensions can add new functionality to a type, but they cannot override existing functionality.

## Extension Syntax

Declare extensions with the `extension` keyword:

extension SomeType {
// new functionality to add to SomeType goes here
}

An extension can extend an existing type to make it adopt one or more protocols. To add protocol conformance, you write the protocol names the same way as you write them for a class or structure:

extension SomeType: SomeProtocol, AnotherProtocol {
// implementation of protocol requirements goes here
}

Adding protocol conformance in this way is described in Adding Protocol Conformance with an Extension.

An extension can be used to extend an existing generic type, as described in Extending a Generic Type. You can also extend a generic type to conditionally add functionality, as described in Extensions with a Generic Where Clause.

NOTE

If you define an extension to add new functionality to an existing type, the new functionality will be available on all existing instances of that type, even if they were created before the extension was defined

## Computed Properties

Extensions can add computed instance properties and computed type properties to existing types. This example adds five computed instance properties to Swift’s built-in `Double` type, to provide basic support for working with distance units:

extension Double {
var km: Double { return self * 1_000.0 }
var m: Double { return self }
var cm: Double { return self / 100.0 }
var mm: Double { return self / 1_000.0 }
var ft: Double { return self / 3.28084 }
}
let oneInch = 25.4.mm
print(“One inch is (oneInch) meters”)
// Prints “One inch is 0.0254 meters”
let threeFeet = 3.ft
print(“Three feet is (threeFeet) meters”)
// Prints “Three feet is 0.914399970739201 meters”

let aMarathon = 42.km + 195.m
print(“A marathon is (aMarathon) meters long”)
// Prints “A marathon is 42195.0 meters long”

NOTE

Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.

## Initializers

Extensions can add new initializers to existing types.

Extensions can add new convenience initializers to a class, but they cannot add new designated initializers or deinitializers to a class. Designated initializers and deinitializers must always be provided by the original class implementation.

NOTE

If you use an extension to add an initializer to a value type that provides default values for all of its stored properties and does not define any custom initializers, you can call the default initializer and memberwise initializer for that value type from within your extension’s initializer.

This would not be the case if you had written the initializer as part of the value type’s original implementation, as described in Initializer Delegation for Value Types.

struct Size {
var width = 0.0, height = 0.0
}
struct Point {
var x = 0.0, y = 0.0
}
struct Rect {
var origin = Point()
var size = Size()
}

let defaultRect = Rect()
let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0),
size: Size(width: 5.0, height: 5.0))

You can extend the `Rect` structure to provide an additional initializer that takes a specific center point and size:

extension Rect {
init(center: Point, size: Size) {
let originX = center.x – (size.width / 2)
let originY = center.y – (size.height / 2)
self.init(origin: Point(x: originX, y: originY), size: size)
}
}

This new initializer starts by calculating an appropriate origin point based on the provided `center` point and `size` value. The initializer then calls the structure’s automatic memberwise initializer `init(origin:size:)`, which stores the new origin and size values in the appropriate properties:

let centerRect = Rect(center: Point(x: 4.0, y: 4.0),
size: Size(width: 3.0, height: 3.0))
// centerRect’s origin is (2.5, 2.5) and its size is (3.0, 3.0)

## Methods

Extensions can add new instance methods and type methods to existing types. The following example adds a new instance method called `repetitions` to the `Int` type:

extension Int {
func repetitions(task: () -> Void) {

// 0과 self사이의 정수를 인덱스로 순환

for _ in 0..<self {
}
}
}

3.repetitions {
print(“Hello!”)
}
// Hello!
// Hello!
// Hello!

Mutating Instance Methods

(참고사항 tumblr #swift #mutating #enum:

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.)

Instance methods added with an extension can also modify (or mutate) the instance itself. Structure and enumeration methods that modify `self` or its properties must mark the instance method as `mutating`, just like mutating methods from an original implementation.

extension Int {
mutating func square() {
self = self * self
}
}
var someInt = 3
someInt.square()
// someInt is now 9

## Subscripts

Extensions can add new subscripts to an existing type.

`123456789[0]` returns `9`

`123456789[1]` returns `8`

extension Int {
subscript(digitIndex: Int) -> Int {
var decimalBase = 1
for _ in 0..<digitIndex {
decimalBase *= 10
}
return (self / decimalBase) % 10
}
}
746381295[0]
// returns 5
746381295[1]
// returns 9
746381295[2]
// returns 2
746381295[8]
// returns 7

746381295[9]
// returns 0, as if you had requested:
0746381295[9]

## Nested Types

Extensions can add new nested types to existing classes, structures, and enumerations:

extension Int {
enum Kind {
case negative, zero, positive
}
var kind: Kind {
switch self {
case 0:
return .zero
case let x where x > 0:
return .positive
default:
return .negative
}
}
}

func printIntegerKinds(_ numbers: [Int]) {
for number in numbers {
switch number.kind {
case .negative:
print(“- ”, terminator: “”)
case .zero:
print(“0 ”, terminator: “”)
case .positive:
print(“+ ”, terminator: “”)
}
}
print(“”)
}
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])
// Prints “+ + – 0 – 0 + ”

NOTE

`number.kind` is already known to be of type `Int.Kind`. Because of this, all of the `Int.Kind` case values can be written in shorthand form inside the `switch` statement, such as `.negative` rather than `Int.Kind.negative`

## Subscripts

Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without needing separate methods for setting and retrieval. For example, you access elements in an `Array` instance as `someArray[index]` and elements in a `Dictionary` instance as `someDictionary[key]`

You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected based on the type of index value you pass to the subscript. Subscripts are not limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.

(array나 dictionary처럼 index,key를 통해 element에 접근가능하게 하는 방법 제공한다는 이야기 그러나 꼭 index,key (string) 형식이 아니어도 상관없음)

## Subscript Syntax

Subscripts enable you to query instances of a type by writing one or more values in square brackets after the instance name. Their syntax is similar to both instance method syntax and computed property syntax. You write subscript definitions with the `subscript` keyword, and specify one or more input parameters and a return type, in the same way as instance methods. Unlike instance methods, subscripts can be read-write or read-only. This behavior is communicated by a getter and setter in the same way as for computed properties:

subscript(index: Int) -> Int {
get {
// return an appropriate subscript value here
}
set(newValue) {
// perform a suitable setting action here
}
}

The type of `newValue` is the same as the return value of the subscript. As with computed properties, you can choose not to specify the setter’s `(newValue)` parameter. A default parameter called `newValue` is provided to your setter if you do not provide one yourself.

As with read-only computed properties, you can simplify the declaration of a read-only subscript by removing the `get` keyword and its braces:

subscript(index: Int) -> Int {
// return an appropriate subscript value here
}

example of a read-only subscript implementation

struct TimesTable {
let multiplier: Int
subscript(index: Int) -> Int {
return multiplier * index
}
}
let threeTimesTable = TimesTable(multiplier: 3)
print(“six times three is (threeTimesTable[6])”)
// Prints “six times three is 18”

## Subscript Usage

Subscripts are typically used as a shortcut for accessing the member elements in a collection, list, or sequence. You are free to implement subscripts in the most appropriate way for your particular class or structure’s functionality.

For example, Swift’s `Dictionary` type

var numberOfLegs = [“spider”: 8, “ant”: 6, “cat”: 4]
numberOfLegs[“bird”] = 2

## Subscript Options

Subscripts can take any number of input parameters, and these input parameters can be of any type. Subscripts can also return any type. Subscripts can use variadic parameters(정해지지 않은 수의 변수 즉 여러개의 parameters), but they can’t use in-out parameters(값을 받고 계산된 값을 되돌리는 경우) or provide default parameter values.

A class or structure can provide as many subscript implementations as it needs, and the appropriate subscript to be used will be inferred based on the types of the value or values that are contained within the subscript brackets at the point that the subscript is used. This definition of multiple subscripts is known as subscript overloading.

While it is most common for a subscript to take a single parameter, you can also define a subscript with multiple parameters if it is appropriate for your type. The following example defines a `Matrix` structure, which represents a two-dimensional matrix of `Double` values. The `Matrix` structure’s subscript takes two integer parameters:

struct Matrix {
let rows: Int, columns: Int
var grid: [Double]
init(rows: Int, columns: Int) {
self.rows = rows
self.columns = columns

// 첫번째 파라미터는 값, 두번째 파라미터는 array 요소의 갯수

// 참조

grid = Array(repeating: 0.0, count: rows * columns)
}
func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}
subscript(row: Int, column: Int) -> Double {
get {
assert(indexIsValid(row: row, column: column), “Index out of range”)
return grid[(row * columns) + column]
}
set {
assert(indexIsValid(row: row, column: column), “Index out of range”)
grid[(row * columns) + column] = newValue
}
}
}

var matrix = Matrix(rows: 2, columns: 2)

matrix[0, 1] = 1.5
matrix[1, 0] = 3.2

func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}

let someValue = matrix[2, 2]
// this triggers an assert, because [2, 2] is outside of the matrix bounds

## Subscripts

Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without needing separate methods for setting and retrieval. For example, you access elements in an `Array` instance as `someArray[index]` and elements in a `Dictionary` instance as `someDictionary[key]`

You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected based on the type of index value you pass to the subscript. Subscripts are not limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.

(array나 dictionary처럼 index,key를 통해 element에 접근가능하게 하는 방법 제공한다는 이야기 그러나 꼭 index,key (string) 형식이 아니어도 상관없음)

## Subscript Syntax

Subscripts enable you to query instances of a type by writing one or more values in square brackets after the instance name. Their syntax is similar to both instance method syntax and computed property syntax. You write subscript definitions with the `subscript` keyword, and specify one or more input parameters and a return type, in the same way as instance methods. Unlike instance methods, subscripts can be read-write or read-only. This behavior is communicated by a getter and setter in the same way as for computed properties:

subscript(index: Int) -> Int {
get {
// return an appropriate subscript value here
}
set(newValue) {
// perform a suitable setting action here
}
}

The type of `newValue` is the same as the return value of the subscript. As with computed properties, you can choose not to specify the setter’s `(newValue)` parameter. A default parameter called `newValue` is provided to your setter if you do not provide one yourself.

As with read-only computed properties, you can simplify the declaration of a read-only subscript by removing the `get` keyword and its braces:

subscript(index: Int) -> Int {
// return an appropriate subscript value here
}

example of a read-only subscript implementation

struct TimesTable {
let multiplier: Int
subscript(index: Int) -> Int {
return multiplier * index
}
}
let threeTimesTable = TimesTable(multiplier: 3)
print(“six times three is (threeTimesTable[6])”)
// Prints “six times three is 18”

## Subscript Usage

Subscripts are typically used as a shortcut for accessing the member elements in a collection, list, or sequence. You are free to implement subscripts in the most appropriate way for your particular class or structure’s functionality.

For example, Swift’s `Dictionary` type

var numberOfLegs = [“spider”: 8, “ant”: 6, “cat”: 4]
numberOfLegs[“bird”] = 2

## Subscript Options

Subscripts can take any number of input parameters, and these input parameters can be of any type. Subscripts can also return any type. Subscripts can use variadic parameters(정해지지 않은 수의 변수 즉 여러개의 parameters), but they can’t use in-out parameters(값을 받고 계산된 값을 되돌리는 경우) or provide default parameter values.

A class or structure can provide as many subscript implementations as it needs, and the appropriate subscript to be used will be inferred based on the types of the value or values that are contained within the subscript brackets at the point that the subscript is used. This definition of multiple subscripts is known as subscript overloading.

While it is most common for a subscript to take a single parameter, you can also define a subscript with multiple parameters if it is appropriate for your type. The following example defines a `Matrix` structure, which represents a two-dimensional matrix of `Double` values. The `Matrix` structure’s subscript takes two integer parameters:

struct Matrix {
let rows: Int, columns: Int
var grid: [Double]
init(rows: Int, columns: Int) {
self.rows = rows
self.columns = columns

// 첫번째 파라미터는 값, 두번째 파라미터는 array 요소의 갯수

// 참조

grid = Array(repeating: 0.0, count: rows * columns)
}
func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}
subscript(row: Int, column: Int) -> Double {
get {
assert(indexIsValid(row: row, column: column), “Index out of range”)
return grid[(row * columns) + column]
}
set {
assert(indexIsValid(row: row, column: column), “Index out of range”)
grid[(row * columns) + column] = newValue
}
}
}

var matrix = Matrix(rows: 2, columns: 2)

matrix[0, 1] = 1.5
matrix[1, 0] = 3.2

func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}

let someValue = matrix[2, 2]
// this triggers an assert, because [2, 2] is outside of the matrix bounds