Add or subtract numbers from different viewports in Swift4 - swift4

Question

I am new here and would like to ask a question that has been working for me for days. I'm just learning Swift 4 and I've come quite a long way. I really do not know what to do any more, and my books on swift do not help me either.
I have created a small testapp, in which should simply be charged.
There are 5 view controllers. The first one has 4 buttons to get to one of the other 4 and to enter a number there in a text box. This number is then output in the first viewcontroller in a label. The numbers are displayed and even the last entered number is displayed again after a restart of the app.
But now I want to charge off the numbers in the first viewcontroller. How can I fix the code?
My Viewports:
my viewports
code from main viewport:
import UIKit
class ViewController: UIViewController, sendValue1, sendValue2, sendValue3, sendValue4 {
#IBOutlet weak var value1: UILabel!
#IBOutlet weak var value2: UILabel!
#IBOutlet weak var value3: UILabel!
#IBOutlet weak var value4: UILabel!
#IBOutlet weak var calculatedValue1: UILabel! // here i want to see the calculated value like from the label 1-4...value1 + value2 + value3 + value4 = ???
#IBOutlet weak var calculatedValue2: UILabel! // here the same like in claculatedValue1 value but with "-" or "*" or something else...
func value1Data(data: String) {
value1.text = data
UserDefaults.standard.set(value1.text, forKey: "value1")
}
func value2Data(data: String) {
value2.text = data
UserDefaults.standard.set(value2.text, forKey: "value2")
}
func value3Data(data: String) {
value3.text = data
UserDefaults.standard.set(value3.text, forKey: "value3")
}
func value4Data(data: String) {
value4.text = data
UserDefaults.standard.set(value4.text, forKey: "value4")
}
override func viewDidAppear(_ animated: Bool) {
if let lastValue1Data = UserDefaults.standard.object(forKey: "value1") as? String {
value1.text = lastValue1Data
}
if let lastValue2Data = UserDefaults.standard.object(forKey: "value2") as? String {
value2.text = lastValue2Data
}
if let lastValue3Data = UserDefaults.standard.object(forKey: "value3") as? String {
value3.text = lastValue3Data
}
if let LastValue4Data = UserDefaults.standard.object(forKey: "value4") as? String {
value4.text = LastValue4Data
}
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "VC1" {
let SendingVC1: Value1ViewController = segue.destination as! Value1ViewController
SendingVC1.delegate = self
}
if segue.identifier == "VC2" {
let SendingVC2: Value2ViewController = segue.destination as! Value2ViewController
SendingVC2.delegate = self
}
if segue.identifier == "VC3" {
let SendingVC3: Value3ViewController = segue.destination as! Value3ViewController
SendingVC3.delegate = self
}
if segue.identifier == "VC4" {
let SendingVC4: Value4ViewController = segue.destination as! Value4ViewController
SendingVC4.delegate = self
}
}
#IBAction func unwindToView1(_ segue: UIStoryboardSegue) {
}
and the code from one of the other four:
import UIKit
protocol sendValue1 {
func value1Data(data: String)
}
class Value1ViewController: UIViewController {
var delegate: sendValue1? = nil
#IBOutlet weak var textValue1: UITextField!
#IBAction func done(_ sender: Any) {
if delegate != nil {
if textValue1.text != nil {
let data = textValue1.text
delegate?.value1Data(data: data!)
dismiss(animated: true, completion: nil)
}
}
}

Answer 1

why is the result always nil here?
let a = Float(value3.text!) ?? 0
let b = Float(value4.text!) ?? 0
let SUM = a + b
calculatedValue1.text = "\(SUM)" + "m"
No matter what I do, the numbers are not processed ...