How to change the text of a generated label from another function - xcode

Question

So I created my first label without story board today and I am wondering how to change the text of the label when I click a button. The action for the button works just fine. I did create an outlet for the label so I could refer to it out of the method I initially called it in but when I click the button the app crashes.
Here is my code:
import UIKit
import Foundation
class ThirdViewController: UIViewController {
#IBOutlet weak var label1: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
self.view.addBackground()
//label one
var label1 = UILabel(frame: CGRectMake(0, 0, view.bounds.width - 80, 30))
label1.center = CGPointMake(view.bounds.width/2, 20)
label1.textAlignment = NSTextAlignment.Center
label1.text = "Pick the largest number"
label1.font = UIFont(name: font, size: 25)
self.view.addSubview(label1)
//number one
var num1 = UIButton(frame: CGRectMake(0, 0, 100, 100))
num1.center = CGPointMake(view.bounds.width/4, view.bounds.height/4)
num1.setTitle("37", forState: UIControlState.Normal)
num1.addTarget(self, action: "num1Tapped", forControlEvents: UIControlEvents.TouchUpInside)
num1.titleLabel!.font = UIFont(name: font, size: 70)
self.view.addSubview(num1)
}
func num1Tapped(UIButton!){
label1.text = "test"
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
}

Answer 1

There are a few problems with your code.
The property you define on top of class definition #IBOutlet weak var label1: UILabel! is not the same you define in (inside) the method viewDidLoad().
Here, you are creating a new local variable, with the same name of your property, an instance variable!
And this variable you created is only accessible inside the method, that's the scope for which it is defined.
The method num1Tapped(_:) is, as it has already been said here, incorrectly defined.
The action that is triggered by an UIControl can have three different signatures:
action()
action(_:)
action(_: forEvent:)
If you don't need a reference of the button and/or the event that triggered the action, you can just use the first one.
As it is now, and forgetting the wrong definition, when the method num1Tapped(_:) is called, it will try to access the instance variable label1, not the local variable defined in the method viewDidLoad(), which, by the way, is impossible to do it.
So, the next thing to resolve is this instance variable label1.
As you have defined, you must connect this declaration with an Outlet in the storyboard, or you must manually initialize the variable.
This is the code from your example that I used (with some modifications) to change the label when the button is touched, and assuming you are creating the label with code:
class ThirdViewController: UIViewController {
var label1: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
//label one
label1 = UILabel(frame: CGRectMake(0, 0, view.bounds.width - 80, 30))
label1.center = CGPointMake(view.bounds.width/2, 20)
label1.textAlignment = NSTextAlignment.Center
label1.text = "Pick the largest number"
// label1.font = UIFont(name: font, size: 25)
view.addSubview(label1)
//number one
// let num1 = UIButton(frame: CGRectMake(0, 0, 100, 100))
let num1 = UIButton(type: .System)
num1.frame = CGRectMake(0, 0, 100, 100)
print("num1: \(num1)")
num1.center = CGPointMake(self.view.bounds.width/4, self.view.bounds.height/4)
num1.setTitle("37", forState: UIControlState.Normal)
num1.addTarget(self, action: "num1Tapped", forControlEvents: UIControlEvents.TouchUpInside)
// num1.addTarget(self, action: "num1TappedWithParameter:", forControlEvents: UIControlEvents.TouchUpInside)
// num1.addTarget(self, action: "num1TappedWithTwoParameters:event:", forControlEvents: UIControlEvents.TouchUpInside)
// num1.titleLabel!.font = UIFont(name: font, size: 70)
self.view.addSubview(num1)
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
func num1Tapped() {
label1.text = "test"
}
func num1TappedWithParameter(sender: UIButton){
label1.text = "test"
}
func num1TappedWithTwoParameters(sender: UIButton, event: UIEvent) {
print("button: \(sender.description) event: \(event)")
}
}
A few extra notes.
With the code you present I don't understand how it even compiles with the wrong signature of the method num1Tapped(UIButton!). This doesn't compile!
Did you change it (with the Xcode suggestion) to num1Tapped(_ :UIButton!)?
If you did, the app crashes because it can find the method you are calling since you defined as it having no parameters:
num1.addTarget(self, action: "num1Tapped", forControlEvents: UIControlEvents.TouchUpInside)
Another thing I don't understand is how can your button being displayed because you it's no set its type.
Did you define the button num1in storyboard and you (wrongly) tried to access here in the code?
addBackground()is commented since it's not here defined.
The same happens when setting the font.
I add two more methods so you can see how to use them.
Be consistent when writing your code. If you access the variable view without the self, do the same when adding the subviews: view.addSubview(num1), or of course, the opposite, use self in both situations.
I hope this helps with your problem.

Answer 2

Your code is almost correct except of a few small changes:
override func viewDidLoad() {
super.viewDidLoad()
self.view.addBackground()
//label one
var label1 = UILabel(frame: CGRectMake(0, 0, view.bounds.width - 80, 30))
label1.center = CGPointMake(view.bounds.width/2, 20)
label1.textAlignment = NSTextAlignment.Center
label1.text = "Pick the largest number"
label1.font = UIFont(name: font, size: 25)
self.view.addSubview(label1)
//number one
var num1 = UIButton(frame: CGRectMake(0, 0, 100, 100))
num1.center = CGPointMake(view.bounds.width/4, view.bounds.height/4)
num1.setTitle("37", forState: UIControlState.Normal)
num1.addTarget(self, action: "num1Tapped:", forControlEvents: UIControlEvents.TouchUpInside)
num1.titleLabel!.font = UIFont(name: font, size: 70)
self.view.addSubview(num1)
}
func num1Tapped(sender: UIButton!){
label1.text = "test"
}
#user2893289 was correct and so was #Arc676. You need to add the ":" in the action and add sender: UIButton! as the buttons action.