import UIKit

class ViewController: UIViewController {

    @IBOutlet var buttonResponseLabel: UILabel!
    var userIsCurrentlyTyping: Bool = false
    var storedValue: Int = 0;
    var currentOperation = 0
    var currentNumberInLabel = ""

    @IBAction func buttonAppend(_ sender: UIButton)
    {
        let buttonTitle = sender.currentTitle!


        switch buttonTitle
        {
            case "+":
            currentOperation = 1
            storedValue = Int(currentNumberInLabel)!
            buttonResponseLabel.text = ""
            case "-":
            currentOperation = 2
            storedValue = Int(currentNumberInLabel)!
            buttonResponseLabel.text = ""
            case "*":
            currentOperation = 3
            storedValue = Int(currentNumberInLabel)!
            buttonResponseLabel.text = ""
            case "/":
            currentOperation = 4
            storedValue = Int(currentNumberInLabel)!
            buttonResponseLabel.text = ""
            default:


            if buttonTitle == "="
            {
                var finalValue = 0
                print(currentNumberInLabel)
                let secondStoredValue = Int(currentNumberInLabel)!
                if currentOperation == 1
                {
                    finalValue = storedValue + secondStoredValue
                    buttonResponseLabel.text = ""
                    buttonResponseLabel.text = String(finalValue)
                }
                else if currentOperation == 2
                {
                    finalValue = storedValue - secondStoredValue
                    buttonResponseLabel.text = ""
                    buttonResponseLabel.text = String(finalValue)
                }
                else if currentOperation == 3
                {
                    finalValue = storedValue * secondStoredValue
                    buttonResponseLabel.text = ""
                    buttonResponseLabel.text = String(finalValue)
                }
                else if currentOperation == 4
                {
                    finalValue = storedValue / secondStoredValue
                    buttonResponseLabel.text = ""
                    buttonResponseLabel.text = String(finalValue)
                }
            }
            else
            {
                if userIsCurrentlyTyping
                {
                    currentNumberInLabel = buttonResponseLabel.text! + buttonTitle
                    buttonResponseLabel.text = currentNumberInLabel
                }
                else
                {
                    userIsCurrentlyTyping = true
                    buttonResponseLabel.text = ""
                    buttonResponseLabel.text = buttonTitle
                }

            }

            print("(currentOperation) (buttonTitle)")
        }

    }

}

import UIKit

class ViewController: UIViewController {

    struct Operator {
        var stringRepresentation: String
        var `operator`: (Int, Int) -> Int //quotes required  as operator is a keyword
    }

    var operators = ["+":Operator(stringRepresentation:"+", operator:+),
                 "-":Operator(stringRepresentation:"-", operator:-),
                 "*":Operator(stringRepresentation:"*", operator:*),
                 "/":Operator(stringRepresentation:"/", operator:/)]
    @IBOutlet var buttonResponseLabel: UILabel!
    var userIsCurrentlyTyping: Bool = false
    var storedValue: Int = 0;
    var currentOperation: Operator?
    var currentNumberInLabel = ""
    var buttonTitle = ""

    @IBAction func buttonAppend(_ sender: UIButton) {
        buttonTitle = sender.currentTitle!

        if let chosenOperator = operators[buttonTitle] {
            currentOperation = chosenOperator
            storedValue = Int(currentNumberInLabel)!
            buttonResponseLabel.text = ""
        } else if buttonTitle == "=" {
            showResult()
        } else if userIsCurrentlyTyping {
            currentNumberInLabel = buttonResponseLabel.text! + buttonTitle
            buttonResponseLabel.text = currentNumberInLabel
        } else {
            userIsCurrentlyTyping = true
            buttonResponseLabel.text = buttonTitle
        }

        print("(currentOperation) (buttonTitle)")
    }

    func showResult() {
        print(currentNumberInLabel)
        guard let currentOperator = currentOperation else {
            print("This should never happen. Fail silently")
            return
        }
        operate(withOperator: currentOperator.operator)
    }

    func operate(withOperator op: (Int, Int) -> Int){
        let secondStoredValue = Int(currentNumberInLabel)!
        let finalValue = op(storedValue, secondStoredValue)
        buttonResponseLabel.text = String(finalValue)
    }
}

enum CalculatorOperation {

    case add
    case subtract
    case multiply
    case divide

    init?(from buttonTitle: String) {
        switch buttonTitle {
        case "+": self = .add
        case "-": self = .subtract
        case "*": self = .multiply
        case "/": self = .divide
        default: return nil
        }
    }

    func apply(to left: Int, and right: Int) -> Int {

        switch self {
        case .add:
            return left + right

        case .subtract:
            return left - right

        case .multiply:
            return left * right

        case .divide:
            return left / right
        }
    }
}

class ViewController: UIViewController {

    @IBOutlet var inputLabel: UILabel!

    var firstNumber: Int = 0
    var currentOperation: CalculatorOperation? = nil

    // connect 1,2,3,4,5,6,7,8,9,0 buttons to this action
    @IBAction func updateNumber(_ numberButton: UIButton) {

        // update the input label with whatever number has been pressed
        let numberTitle = numberButton.currentTitle!
        inputLabel.text = inputLabel.text! + numberTitle
    }

    // connect '+', '-', '*', '/' buttons to this action
    @IBAction func updateOperation(_ operationButton: UIButton) {

        // update the operation based on the key pressed
        let operationTitle = operationButton.currentTitle!
        currentOperation = CalculatorOperation(from: operationTitle)

        // save the number shown in the input as the first number
        let currentinput = inputLabel.text!
        firstNumber = Int(currentinput)!

        // clear out the input label so it's ready for new number to be entered
        inputLabel.text = ""
    }

    // connect '=' button to this action
    @IBAction func performCalculation(_ sender: UIButton) {

        // get the second number from whatever is currently in the input label
        let currentinput = inputLabel.text!
        let secondNumber = Int(currentinput)!

        let finalValue = currentOperation!.apply(to: firstNumber, and: secondNumber)

        // update input label with the calculated value
        inputLabel.text = String(finalValue)
    }
}

enter code here     import UIKit

override func viewDidLoad() {
    super.viewDidLoad()

}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()

}

@IBOutlet weak var current: UITextField!

@IBOutlet weak var precentExamWorth: UITextField!
@IBOutlet weak var desired: UITextField!

{
   var gradeNeeded = 0
  var precentExamWorth = 0.15
var desired = 0
   var current = 0




        var x = 0 - (0.85 * 0) / 0.15