Parsing a n-dimensional array in swift

Question

I'm trying to parse a multidimensional array that is n levels deep using swift. An example of the input, that is 3 levels deep, is:

[+, 5, 4, ( "+", 4, ( "-", 7, 3 ) )] 

The goal of the code is to take the item at arr[0] and do that operation to the other items in that level of the array.

3 nested for loops seems like the way to go for this particular input set, but I can't figure out how to write code that will work for arrays that are n levels deep.

Thanks.

Answer 1

Look like you are building an RPN calculator. Instead of nested if , you can use recursion:

func evaluate (stack: [AnyObject]) -> Double {
    func apply(op: String, _ operand1: Double, _ operand2: Double) -> Double {
        switch op {
        case "+": return operand1 + operand2
        case "-": return operand1 - operand2
        case "x": return operand1 * operand2
        case "/": return operand1 / operand2
        default:
            print("Invalid operator: \(op)")
            return Double.NaN
        }
    }

    guard let op = stack[0] as? String else {
        fatalError("stack must begin with an operator")
    }

    switch (stack[1], stack[2]) {
    case (let operand1 as [AnyObject], let operand2 as [AnyObject]):
        return apply(op, evaluate(operand1), evaluate(operand2))
    case (let operand1 as [AnyObject], let operand2 as Double):
        return apply(op, evaluate(operand1), operand2)
    case (let operand1 as Double, let operand2 as [AnyObject]):
        return apply(op, operand1, evaluate(operand2))
    case (let operand1 as Double, let operand2 as Double):
        return apply(op, operand1, operand2)
    default:
        print("I don't know how to handle this: \(stack)")
        return Double.NaN
    }
}

let rpnStack = ["+", 5, ["+", 4, [ "-", 7, 3]]]
let result = evaluate(rpnStack)
print(result)    // 13

This obviously assumes the expression tree at each level contains exact 3 nodes.