initialization of immutable value 'rate' was never used - swift

Question

I am brand new to Swift and Xcode and I am trying to build a financial calculator for Mac and I have encountered two issues.
The first issue is that in my code, it says that I initialized a value, but never used it. I believe that I have actually done that, but I keep getting the error. Here is my entire code for the class I'm working on:
import Cocoa
class PresentValueController: NSViewController {
#IBOutlet weak var answer_label: NSTextField!
#IBOutlet weak var payment_field: NSTextField!
#IBOutlet weak var rate_field: NSTextField!
#IBOutlet weak var periods_field: NSTextField!
#IBOutlet weak var compounding_popup: NSPopUpButton!
override func viewDidLoad() {
super.viewDidLoad()
// Do view setup here.
}
#IBAction func popupValueChanged(_ sender: NSPopUpButton) {
let rate = (rate_field.integerValue)/sender.selectedTag()
return
}
#IBAction func calculate_button(_ sender: Any) {
let payment = (payment_field.integerValue)
let present_value = (payment / (1 + rate)^12)
answer_label.stringValue = "$\(present_value)"
}
}
As you can see, I have defined rate and tried to use it, but I keep getting the error.
My next question has to do with aesthetics. This is what I want my output to look like, but this is what ends up happening when I comment out the errors and run the code. Could I please get some help?
Thanks!!!

Answer 1

The problem is here
let rate = (rate_field.integerValue)/sender.selectedTag()
you don't use , you may think it's visible inside the other method but it's not , also remove the return
//
#IBAction func calculate_button(_ sender: Any) {
let rate = (rate_field.integerValue)/ popButton.selectedTag()
let payment = (payment_field.integerValue)
let present_value = (payment / (1 + rate)^12)
answer_label.stringValue = "$\(present_value)"
}
and declare
#IBOutlet weak var popButton: NSPopUpButton!
if it's not already
#IBOutlet weak var compounding_popup: NSPopUpButton!

Answer 2

Your rate variable is local to popupValueChanged(_:), and your reference to rate in calculate_button(_:) is a reference to an undefined variable.
You could save rate as an instance variable of your PresentValueController objects, but I would advise against it, because you would be duplicating data, and it's easy for it to go out of sync as the complexity of your app grows.
Instead, I recommend you just compute the rate within your calculate_button(_:) function, and drop the popupValueChanged(_:) function completely:
class PresentValueController: NSViewController {
#IBOutlet weak var answerLabel: NSTextField!
#IBOutlet weak var paymentField: NSTextField!
#IBOutlet weak var rateField: NSTextField!
#IBOutlet weak var periodsField: NSTextField!
#IBOutlet weak var compoundingPopup: NSPopUpButton!
#IBAction func calculateButton(_ sender: NSButton) {
let rate = rateField.integerValue / sender.selectedTag()
let payment = paymentField.doubleValue
let presentValue = payment / pow(1 + Double(rate), 12)
let formatter: NumberFormatter = {
let f = NumberFormatter()
f.locale = NSLocale.current
f.numberStyle = .currency
return f
}()
answerLabel.stringValue = formatter.string(from: NSNumber(value: presentValue))!
}
}

Answer 3

You need to understand the concepts of scope and local variables.
A variable (or a let constant) you define inside a function only exists inside the function. (For the rest of this post I'm going to use the term "variable", "immutable variable" and "let constant" interchangeably, even though that's a bit sloppy.)
Your function popupValueChanged() creates a local variable rate that only exists inside the function. As soon as you exit the function, the local variables you define inside the function go out of scope and cease to exist.
Based on the code you posted I would expect your function calculate_button() to have another error about an undefined expression rate.
Consider this code:
class AClass {
let rate = 3
func foo() {
let rate = 6
print("inside function, rate =", rate)
print("inside function, self.rate = ", self.rate)
}
}
let anObject = AClass()
anObject.foo()
print("outside function, anObject.rate = ", anObject.rate)
print("rate =", rate) //This will give an error.
(pretend it's entered into a playground)
The code will do the following:
Create an instance of AClass. The initialization of the AClass object will create an immutable instance variable rate with a value of 3.
Then we call the AClass object's foo() method, which defines a different, local variable, also named rate, and assigns the value 6 to that local variable.
The output of that code will be
inside function, rate = 6
inside function, anObject.rate = 3
outside function, anObject.rate = 3
Inside the function, the local variable rate hides the instance variable rate, so referring to rate inside the function refers to the local immutable variable. Since there are 2 different variables named rate defined at different scopes, we have to use self.rate to refer to the instance variable rate inside the function.
Outside the foo() function the function's local variable rate no longer exists so rate refers to the instance variable rate, who's value has always been 3.
The line:
print("rate =", rate) //This will give an error.
Will give a compile error since outside the function there is no variable at that scope with the name rate.