无法转换“字符串”类型的值？强制输入“NSString”

Question

我正在尝试为各种数学公式制作这个计算器，但我被困在这一点上。我在关注这个tutorial

这是我的代码：

import UIKit

class pythagorasViewController: UIViewController {


@IBOutlet weak var aLabel: UILabel!
@IBOutlet weak var bLabel: UILabel!
@IBOutlet weak var aField: UITextField!
@IBOutlet weak var bField: UITextField!
@IBOutlet weak var answerLabel: UILabel!
@IBAction func calculateButton(_ sender: UIButton) {
    var a = (aField.text as NSString).floatValue
    var b = (bField.text as NSString).floatValue
    var answer = sqrt(a*a + b*b)
    answerLabel.text = "\(answer)"
}

override func viewDidLoad() {
    super.viewDidLoad()

    // Do any additional setup after loading the view.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

}

我得到错误的部分是：

var a = (aField.text as NSString).floatValue
var b = (bField.text as NSString).floatValue

Answer 1

在可能的情况下，首选 let 而不是 var。您不需要使用 NSString。您可以将 String 转换为 Float?。您需要解开作为字符串的文本属性吗？ （如果您对变量选项的类型有疑问，请单击它会向您显示）和浮点数？转换：

func calculateButton(_ sender: UIButton) {
    guard let aText = aField.text,
          let bText = bField.text,
          let a = Float(aText),
          let b = Float(bText) else {
        return
    }
    let answer = sqrt(a*a + b*b)
    answerLabel.text = "\(answer)"
}