Closures in Swift

Static vs class functions/variables in Swift classes?

Functions

Swift’s unified function syntax is flexible enough to express anything from a function with no parameter names to method with names and argument labels for each parameter. Parameters can provide default values.

Every function in Swift has a type, consisting of the function’s parameter types and return type. Functions can also be written within other functions to encapsulate useful functionality within a nested function scope.

Defining and Calling Functions

When you define a function, you can optionally define one or more named, typed values that the function takes as input, known as parameters. You can also optionally define a type of value that the function will pass back as output when it is done, known as its return type.

To use a function, you “call” that function with its name and pass it input values. A function’s arguments must always be provided in the same order as the function’s parameter list.

func greet(person: String) -> String {
   let greeting = “Hello, ” + person + “!”
   return greeting
}

NOTE

The print(_:separator:terminator:) function doesn’t have a label for its first argument, and its other arguments are optional because they have a default value.

Function Parameters and Return Values

Functions Without Parameters

func sayHelloWorld() -> String {
   return “hello, world”
}
print(sayHelloWorld())
// Prints “hello, world”

Functions With Multiple Parameters

func greet(person: String, alreadyGreeted: Bool) -> String {
   if alreadyGreeted {
       return greetAgain(person: person)
   } else {
       return greet(person: person)
   }
}
print(greet(person: “Tim”, alreadyGreeted: true))
// Prints “Hello again, Tim!”

greet(person:alreadyGreeted:) 함수와 

greet(person:) 는 이름은 같으나 받아들이는 parameters 가 다르므로 다른 함수이다.

Functions Without Return Values

func greet(person: String) {
   print(“Hello, (person)!”)
}
greet(person: “Dave”)
// Prints “Hello, Dave!”

Because it does not need to return a value, the function’s definition does not include the return arrow (->) or a return type.

NOTE

Strictly speaking, this version of the greet(person:) function does still return a value, even though no return value is defined. Functions without a defined return type return a special value of type Void. This is simply an empty tuple, which is written as ()

.The return value of a function can be ignored when it is called:

func printAndCount(string: String) -> Int {
   print(string)
   return string.count
}
func printWithoutCounting(string: String) {
   let _ = printAndCount(string: string)
}
printAndCount(string: “hello, world”)
// prints “hello, world” and returns a value of 12
printWithoutCounting(string: “hello, world”)
// prints “hello, world” but does not return a value

NOTE

Return values can be ignored, but a function that says it will return a value must always do so. A function with a defined return type cannot allow control to fall out of the bottom of the function without returning a value, and attempting to do so will result in a compile-time error.

Functions with Multiple Return Values

func minMax(array: [Int]) -> (min: Int, max: Int) {
   var currentMin = array[0]
   var currentMax = array[0]
   for value in array[1..<array.count] {
       if value < currentMin {
           currentMin = value
       } else if value > currentMax {
           currentMax = value
       }
   }
   return (currentMin, currentMax)
}

let bounds = minMax(array: [8, -6, 2, 109, 3, 71])
print(“min is (bounds.min) and max is (bounds.max)”)
// Prints “min is -6 and max is 109”

Note 

that the tuple’s members do not need to be named at the point that the tuple is returned from the function, because their names are already specified as part of the function’s return type.

Optional Tuple Return Types

you can use an optional tuple return type to reflect the fact that the entire tuple can be nil. You write an optional tuple return type by placing a question mark after the tuple type’s closing parenthesis, such as (Int, Int)?or (String, Int, Bool)?.

NOTE

An optional tuple type such as (Int, Int)? is different from a tuple that contains optional types such as (Int?, Int?). With an optional tuple type, the entire tuple is optional, not just each individual value within the tuple.

func minMax(array: [Int]) -> (min: Int, max: Int)? {
   if array.isEmpty { return nil }
   var currentMin = array[0]
   var currentMax = array[0]
   for value in array[1..<array.count] {
       if value < currentMin {
           currentMin = value
       } else if value > currentMax {
           currentMax = value
       }
   }
   return (currentMin, currentMax)
}

You can use optional binding to check whether this version of the minMax(array:) function returns an actual tuple value or nil:

if let bounds = minMax(array: [8, -6, 2, 109, 3, 71]) {
   print(“min is (bounds.min) and max is (bounds.max)”)
}
// Prints “min is -6 and max is 109”

Function Argument Labels and Parameter Names

Each function parameter has both an argument label and a parameter name. The argument label is used when calling the function; each argument is written in the function call with its argument label before it. The parameter name is used in the implementation of the function. By default, parameters use their parameter name as their argument label.

func someFunction(firstParameterName: Int, secondParameterName: Int) {
   // In the function body, firstParameterName and secondParameterName
   // refer to the argument values for the first and second parameters.
}
someFunction(firstParameterName: 1, secondParameterName: 2)

You write an argument label before the parameter name, separated by a space:

func someFunction(argumentLabel parameterName: Int) {
   // In the function body, parameterName refers to the argument value
   // for that parameter.
}

argument label 과  parameter name 를 구분해서 사용하는 예시

func greet(person: String, from hometown: String) -> String {
   return “Hello (person)!  Glad you could visit from (hometown).”
}
print(greet(person: “Bill”, from: “Cupertino”))
// Prints “Hello Bill!  Glad you could visit from Cupertino.”

Omitting Argument Labels

If you don’t want an argument label for a parameter, write an underscore (_) instead of an explicit argument label for that parameter.

func someFunction(_ firstParameterName: Int, secondParameterName: Int) {
   // In the function body, firstParameterName and secondParameterName
   // refer to the argument values for the first and second parameters.
}
someFunction(1, secondParameterName: 2)

Default Parameter Values

func someFunction(parameterWithoutDefault: Int, parameterWithDefault: Int = 12) {
   // If you omit the second argument when calling this function, then
   // the value of parameterWithDefault is 12 inside the function body.
}
someFunction(parameterWithoutDefault: 3, parameterWithDefault: 6) // parameterWithDefault is 6
someFunction(parameterWithoutDefault: 4) // parameterWithDefault is 12

Variadic Parameters

A variadic parameter accepts zero or more values of a specified type. You use a variadic parameter to specify that the parameter can be passed a varying number of input values when the function is called. Write variadic parameters by inserting three period characters (...) after the parameter’s type name.

The values passed to a variadic parameter are made available within the function’s body as an array of the appropriate type.

func arithmeticMean(_ numbers: Double…) -> Double {
   var total: Double = 0
   for number in numbers {
       total += number
   }
   return total / Double(numbers.count)
}
arithmeticMean(1, 2, 3, 4, 5)
// returns 3.0, which is the arithmetic mean of these five numbers
arithmeticMean(3, 8.25, 18.75)
// returns 10.0, which is the arithmetic mean of these three numbers

NOTE

A function may have at most one variadic parameter.

In-Out Parameters

Function parameters are constants by default. Trying to change the value of a function parameter from within the body of that function results in a compile-time error. If you want a function to modify a parameter’s value(전달된 variable형식의 argument값을 함수내에서 처리한후 외부에서도 variable의 변화값이 유지되게 하는 경우 ), and you want those changes to persist after the function call has ended, define that parameter as an in-out parameter instead.

You write an in-out parameter by placing the inout keyword right before a parameter’s type. 

You can only pass a variable as the argument for an in-out parameter. You cannot pass a constant or a literal value as the argument, because constants and literals cannot be modified. You place an ampersand (&) directly before a variable’s name when you pass it as an argument to an in-out parameter, to indicate that it can be modified by the function.

NOTE

In-out parameters cannot have default values, and variadic parameters cannot be marked as inout.

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”

NOTE

In-out parameters are not the same as returning a value from a function. The swapTwoInts example above does not define a return type or return a value, but it still modifies the values of someInt and anotherInt. In-out parameters are an alternative way for a function to have an effect outside of the scope of its function body.

Function Types

Every function has a specific function type, made up of the parameter types and the return type of the function.

func addTwoInts(_ a: Int, _ b: Int) -> Int {
   return a + b
}
func multiplyTwoInts(_ a: Int, _ b: Int) -> Int {
   return a * b
}

The type of both of these functions is (Int, Int) -> Int. This can be read as:

“A function that has two parameters, both of type Int, and that returns a value of type Int.”

Here’s another example, for a function with no parameters or return value:

func printHelloWorld() {
   print(“hello, world”)
}

The type of this function is () -> Void, or “a function that has no parameters, and returns Void.”

Using Function Types

You use function types just like any other types in Swift. For example, you can define a constant or variable to be of a function type and assign an appropriate function to that variable:

var mathFunction: (Int, Int) -> Int = addTwoInts

This can be read as:

“Define a variable called mathFunction, which has a type of ‘a function that takes two Int values, and returns an Int value.’ Set this new variable to refer to the function called addTwoInts.”

The addTwoInts(_:_:) function has the same type as the mathFunction variable, and so this assignment is allowed by Swift’s type-checker.

You can now call the assigned function with the name mathFunction:

print(“Result: (mathFunction(2, 3))”)
// Prints “Result: 5”

A different function with the same matching type can be assigned to the same variable, in the same way as for nonfunction types:

mathFunction = multiplyTwoInts
print(“Result: (mathFunction(2, 3))”)
// Prints “Result: 6”

As with any other type, you can leave it to Swift to infer the function type when you assign a function to a constant or variable:

let anotherMathFunction = addTwoInts
// anotherMathFunction is inferred to be of type (Int, Int) -> Int

Function Types as Parameter Types

You can use a function type such as (Int, Int) -> Int as a parameter type for another function. This enables you to leave some aspects of a function’s implementation for the function’s caller to provide when the function is called.

Here’s an example to print the results of the math functions from above:

func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) {
   print(“Result: (mathFunction(a, b))”)
}
printMathResult(addTwoInts, 3, 5)
// Prints “Result: 8”

This example defines a function called printMathResult(_:_:_:), which has three parameters. The first parameter is called mathFunction, and is of type (Int, Int) -> Int. You can pass any function of that type as the argument for this first parameter. The second and third parameters are called a and b, and are both of type Int. These are used as the two input values for the provided math function.

When printMathResult(_:_:_:) is called, it is passed the addTwoInts(_:_:) function, and the integer values 3and 5. It calls the provided function with the values 3 and 5, and prints the result of 8.

The role of printMathResult(_:_:_:) is to print the result of a call to a math function of an appropriate type. It doesn’t matter what that function’s implementation actually does—it matters only that the function is of the correct type. This enables printMathResult(_:_:_:) to hand off some of its functionality to the caller of the function in a type-safe way.

(printMathResult 라는 wrapper 함수를 만들고 함수의 타입만 맞는다면 다양한 함수와 변수를 받아드리고 가변적인 결과를 만들어 낼수 있다는 것을 보여주는 예시 )

Function Types as Return Types

You can use a function type as the return type of another function. You do this by writing a complete function type immediately after the return arrow (->) of the returning function.

The next example defines two simple functions called stepForward(_:) and stepBackward(_:). The stepForward(_:) function returns a value one more than its input value, and the stepBackward(_:) function returns a value one less than its input value. Both functions have a type of (Int) -> Int:

func stepForward(_ input: Int) -> Int {
   return input + 1
}
func stepBackward(_ input: Int) -> Int {
   return input – 1
}

Here’s a function called chooseStepFunction(backward:), whose return type is (Int) -> Int. The chooseStepFunction(backward:) function returns the stepForward(_:) function or the stepBackward(_:) function based on a Boolean parameter called backward:

func chooseStepFunction(backward: Bool) -> (Int) -> Int {
   return backward ? stepBackward : stepForward
}

You can now use chooseStepFunction(backward:) to obtain a function that will step in one direction or the other:

var currentValue = 3
let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
// moveNearerToZero now refers to the stepBackward() function

The preceding example determines whether a positive or negative step is needed to move a variable called currentValue progressively closer to zero. currentValue has an initial value of 3, which means that currentValue > 0 returns true, causing chooseStepFunction(backward:) to return the stepBackward(_:)function. A reference to the returned function is stored in a constant called moveNearerToZero.

Now that moveNearerToZero refers to the correct function, it can be used to count to zero:

print(“Counting to zero:”)
// Counting to zero:
while currentValue != 0 {
   print(“(currentValue)… ”)
   currentValue = moveNearerToZero(currentValue)
}
print(“zero!”)
// 3…
// 2…
// 1…
// zero!

(상황에 따라 리턴하는 함수가 달라지는 wrapper함수를 만드는 과정을 보여준다. )

Nested Functions

All of the functions you have encountered so far in this chapter have been examples of global functions, which are defined at a global scope. You can also define functions inside the bodies of other functions, known as nested functions.

Nested functions are hidden from the outside world by default, but can still be called and used by their enclosing function. An enclosing function can also return one of its nested functions to allow the nested function to be used in another scope.

You can rewrite the chooseStepFunction(backward:) example above to use and return nested functions:

func chooseStepFunction(backward: Bool) -> (Int) -> Int {
   func stepForward(input: Int) -> Int { return input + 1 }
   func stepBackward(input: Int) -> Int { return input – 1 }
   return backward ? stepBackward : stepForward
}
var currentValue = -4
let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
// moveNearerToZero now refers to the nested stepForward() function
while currentValue != 0 {
   print(“(currentValue)… ”)
   currentValue = moveNearerToZero(currentValue)
}
print(“zero!”)
// -4…
// -3…
// -2…
// -1…
// zero!

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