Swift에서 꼬리재귀 사용기 (Tail Recursion)

func fac(_ n: Int) -> Int {
guard n > 0 else { return 1 }
return n * fac(n-1)
}
fac(4)
➔ 4 * fac(3)
➔ 4 * 3 * fac(2)
➔ 4 * 3 * 2 * fac(1)
➔ 4 * 3 * 2 * 1 * fac(0)
➔ 4 * 3 * 2 * 1 * 1
➔ 4 * 3 * 2 * 1
➔ 4 * 3 * 2
➔ 4 * 6
➔ 24

func fac(_ n: Int) -> Int {
func fac(_ index: Int, _ answer: Int) -> Int {
if index == 1 {
return answer
}
return fac(index-1, answer * index)
}
return fac(n,1)
}
fac(4)
➔ fac(4,1)
➔ fac(3,4)
➔ fac(2,12)
➔ fac(1,24)
➔ 24
.

func fib(_ n: Int) -> Int {
if n == 0 { return 0 }
if n == 1 { return 1 }
return fib(n-1) + fib(n-2)
}
fib(5)
fib(4) fib(3)
fib(1)
1
fib(2)
fib(0)
0
fib(1)
1
fib(2)
fib(0)
0
fib(1)
1
fib(3)
fib(1)
1
fib(2)
fib(0)
0
fib(1)
1

func fib(_ n: Int) -> Int {
func fib(_ a: Int, _ b: Int, _ n: Int) -> Int {
if n == 0 { return a }
return fib(b, a+b, n-1)
}
return fib(0,1,n)
}
fib(6)
➔ fib(0,1,6)
➔ fib(1,1,5)
➔ fib(1,2,4)
➔ fib(2,3,3)
➔ fib(3,5,2)
➔ fib(5,8,1)
➔ fib(8,13,0)
➔ 8

fac(4)
➔ fac(4,1)
➔ fac(3,4)
➔ fac(2,12)
➔ fac(1,24)
➔ 24
fac(4)
➔ 4 * fac(3)
➔ 4 * 3 * fac(2)
➔ 4 * 3 * 2 * fac(1)
➔ 4 * 3 * 2 * 1 * fac(0)
➔ 4 * 3 * 2 * 1 * 1
➔ 4 * 3 * 2 * 1
➔ 4 * 3 * 2
➔ 4 * 6
➔ 24

: https://brunch.co.kr/@sunghokimnxag/5
8
• 1 .
• index 7 (7,14,21…) 7 (7,17,27) .
• 1, 2, 3, 4, 5, 6, [7], 6, 5, 4, 3, 2, 1, [0], 1, 2, [3], 2, 1, 0, [-1], 0, 1,…
• pingpong(x) .
• For Loop Array .
• Assignment , .
• String .
pingpong(8) = 6
pingpong(22) = 0
pingpong(68) = 2
pingpong(100) = 2

func has7(_ index: Int) -> Bool {
if index % 7 == 0 {
return true
}
if (index - 7) % 10 == 0 {
return true
}
if (index / 10) > 1 {
return has7(index/10)
}
return false
}
has7(21)
➔ true
has7(723)
➔ has7(72)
➔ has7(7)
➔ true
has7(123)
➔ has7(12)
➔ has7(1)
➔ false

unc pingpong(_ index: Int) -> Int {
func pingpong(_ index: Int) -> (Int, Int) {
if index == 1 {
return (1, 1)
}
switch pingpong( index - 1 ) {
case let (answer, direction) :
if has7(index) {
return (answer+direction, -direction)
} else {
return (answer+direction, direction)
}
}
}
return pingpong(index).0
pingpong(14) ➔ (pingpong(13).answer-1, 1)
pingpong(13) ➔ (pingpong(12).answer-1, -1)
pingpong(10) ➔ (pingpong( 9).answer-1, -1)
pingpong(7) ➔ (pingpong( 6).answer+1, -1)
pingpong(6) ➔ (pingpong( 5).answer+1, 1)
pingpong(1) ➔ (1,1)

nc pingpong(_ count: Int) -> Int {
func pp(_ count: Int, _ index: Int, _ direction: Int, _ answer: Int) -> Int {
if count == index {
return answer
}
if has7(index+1) {
return pp(count, index+1, -direction, answer+direction)
} else {
return pp(count, index+1, direction, answer+direction)
}
}
return pp(count, 1, 1, 1)
pp(14, index: 1, direction: 1,answer: 1)
pp(14, index: 6, direction:-1,answer: 6)
pp(14, index:10, direction:-1,answer: 4)
pp(14, index:13, direction: 1,answer: 1)
➔ 0

let array = [1,3,5,23,15]
func sum(array: [Int], initial: Int) -> Int {
guard let head = array.first else { return initial }
let tail = Array(array.dropFirst())
return sum(array: tail, initial: initial + head)
}
sum(array: array, initial: 0)
➔ sum(array: [1,3,5,23,15], initial: 0)
➔ sum(array: [3,5,23,15], initial: 1)
➔ sum(array: [5,23,15], initial: 4)
➔ sum(array: [23,15], initial: 9)
➔ sum(array: [15], initial: 32)
➔ sum(array: [], initial: 47)
➔ 47

func tailRecursive(array: [Int] ,
initial: Int,
eachOperation: (Int, Int) -> Int ) -> Int {
guard let head = array.first else { return initial }
let tail = Array(array.dropFirst())
return tailRecursive(array: tail,
initial: eachOperation(head, initial),
eachOperation: eachOperation)
}
tailRecursive(array: array,
initial: 0,
eachOperation:{(element: Int, answer: Int)->Int in
element + answer
})
➔ 47

extension Array {
func tailRecursive<Result>(
_ initial: Result,
eachOperation: (Result, Element) -> Result ) -> Result {
guard let head = self.first else { return initial }
let tail = Array(self.dropFirst())
return tail.tailRecursive(eachOperation(initial, head),
eachOperation: eachOperation)
}
}
array.tailRecursive(0, eachOperation: + )
array.tailRecursive(0) { (element: Int, answer: Int) -> Int in
return element + answer
}
array.reduce(0) { (element: Int, answer) -> Int in
return element + answer
}

func searchTopViewController(viewController: UIViewController?) -> UIViewController? {
guard let viewController = viewController else { return nil }
switch viewController {
case let viewController as UITabBarController:
return searchTopViewController(viewController: viewController.selectedViewController)
case let viewController as UINavigationController:
return searchTopViewController(viewController: viewController.viewControllers.last)
case let viewController where viewController.presentedViewController != nil:
return searchTopViewController(viewController:viewController.presentedViewController)
default:
return viewController
}
}

Swift에서 꼬리재귀 사용기 (Tail Recursion)

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