Stack implementation in Swift

Question

I'm new to Swift and iOS programming.

I'm trying to test out a simple algorithm and need an array of Stacks. Don't have to be anything fancy (Stacks of Ints will do).

I got the Stack implementation from The Swift Programming Language documentation:

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

The count and show functions are my contribution. But when I try to declare an array of Stacks I get an error...

var lines = IntStack()[5]

"IntStack" does not have a member named subscript

I'm guessing it has something to do with Optionals but can figure out what it is.

Any help?

Answer 1

Details

• Swift 5.1, Xcode 11.3.1

Generic Stack Implementation

Stackable protocol

protocol Stackable {
    associatedtype Element
    func peek() -> Element?
    mutating func push(_ element: Element)
    @discardableResult mutating func pop() -> Element?
}

extension Stackable {
    var isEmpty: Bool { peek() == nil }
}

Stack

struct Stack<Element>: Stackable where Element: Equatable {
    private var storage = [Element]()
    func peek() -> Element? { storage.last }
    mutating func push(_ element: Element) { storage.append(element)  }
    mutating func pop() -> Element? { storage.popLast() }
}

extension Stack: Equatable {
    static func == (lhs: Stack<Element>, rhs: Stack<Element>) -> Bool { lhs.storage == rhs.storage }
}

extension Stack: CustomStringConvertible {
    var description: String { "\(storage)" }
}
    
extension Stack: ExpressibleByArrayLiteral {
    init(arrayLiteral elements: Self.Element...) { storage = elements }
}

Usage

var stack = Stack<Int>()
stack.push(1)
stack.push(2)
stack.push(3)
print(stack.peek())
print(stack.pop())
print(stack)
print(stack == Stack<Int>())
stack = [3,2,1]
print(stack)

Answer 2

There's no problem with what you're doing there - that's just not the syntax for declaring an array. If you want an array of 5 stacks, you can do this:

[IntStack(), IntStack(), IntStack(), IntStack(), IntStack()]

Or, you can initialise the array like this:

Array(count: 5, repeatedValue: IntStack())

Also, you don't need to mark your functions as mutating unless they actually mutate the structure - so count() and show() don't need it.