LESSON 412 OF 415 · PROJECTS: OBJECT-ORIENTED

Stack: Build a MyStack Class (Swift) in Swift

Practice

A stack is a pile where you only touch the top: the last value you put in is the first one you take out (LIFO, last in, first out), like a pile of plates. Build a class MyStack that stores whole numbers in a private var data = [Int]() array and offers four operations.

The methods:

func push(_ value: Int)
func pop() -> Int
func peek() -> Int
func size() -> Int

Rules:

push puts the value on top of the stack. It returns nothing.
pop removes the top value (the one pushed last) and returns it.
peek returns the top value but leaves it on the stack, so the size does not change.
size returns how many values are on the stack. A new stack has size 0, and after popping everything the size is 0 again.
The stack keeps working after it becomes empty: you can push again.
Calling pop or peek on an empty stack is not part of this exercise and the tests never do it. (In Swift, removeLast() and last! on an empty array crash the program.)

You start with: The starter already contains the code that uses your stack (below).

let stack = MyStack()
stack.push(10)
stack.push(20)
stack.push(30)
print("Top: \(stack.peek())")
print("Size: \(stack.size())")
print("Pop: \(stack.pop())")
print("Size: \(stack.size())")
print("Pop: \(stack.pop())")
print("Pop: \(stack.pop())")
print("Empty size: \(stack.size())")
stack.push(7)
print("Top: \(stack.peek())")
print("Size: \(stack.size())")

Expected output:

Top: 30
Size: 3
Pop: 30
Size: 2
Pop: 20
Pop: 10
Empty size: 0
Top: 7
Size: 1

Reading the output: after pushing 10, 20 and 30 the top is 30 and the size 3. Popping gives 30, leaving size 2. Two more pops give 20 and then 10 (last in, first out), the stack is empty (size 0), and after pushing 7 the top is 7 and the size 1.

The top of the stack is the end of the list: adding and removing at the end is fast, and the last value pushed is always the last element.

Arrays have append, removeLast() (which removes and returns the last element) and last (an optional, so write data.last!).

class MyStack {
    private var data = [Int]()

    func push(_ value: Int) {
        data.append(value)
    }

    func pop() -> Int {
        return data.removeLast()
    }

    func peek() -> Int {
        return data.last!
    }

    func size() -> Int {
        return data.count
    }
}

STEP 1 OF 10

Guided

Ctrl + Enter to check

Stack: Build a MyStack Class (Swift) in Swift

Stack Impl in Swift