Swift 错误指令错误购物清单

Question

我有一个问题，当我试图让我的按钮删除我的数组“shoppingList”中的索引时，Xcode 给了我这个错误“EXC_BAD_INSTRUCTION(code=EXC_1386_INVOP,subcode==0*0)”。 请帮助我并告诉我我做错了什么，以便我以后改进。

//
//  ViewController.swift
//  ShoppingList
//
//  Created by Petr Chrastek on 29/03/16.
//  Copyright © 2016 ACS. All rights reserved.
//
class ViewController: UIViewController {
    @IBOutlet weak var labelText: UILabel!
    @IBOutlet weak var label0: UILabel!
    @IBOutlet weak var label1: UILabel!
    @IBOutlet weak var label2: UILabel!
    @IBOutlet weak var label3: UILabel!
    var shoppingList = ["eggs", "milk", "cake", "sugar"]

    @IBAction func remove0(sender: UIButton) {
        shoppingList.removeAtIndex(0)
    }

    @IBAction func remove1(sender: UIButton) {
        shoppingList.removeAtIndex(1)
    }

    @IBAction func remove2(sender: UIButton) {
        shoppingList.removeAtIndex(2)
    }

    @IBAction func remove3(sender: UIButton) {
        shoppingList.removeAtIndex(3)
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        let str: String? = shoppingList[0]
        let str1: String? = shoppingList[1]
        let str2: String? = shoppingList[2]
        let str3: String? = shoppingList[3]
        let count = shoppingList.count
        labelText.text? = "you are missing \(count) items"
        if str != nil {
            label0.text? = "\(str)"
        } else {
            label0.text? = "empty"
        }
        if str1 != nil {
            label1.text? = "\(str1)"
        } else {
            label1.text? = "empty"
        }
        if str2 != nil {
            label2.text? = "\(str2)"
        } else {
            label2.text? = "empty"
        }
        if str3 != nil {
            label3.text? = "\(str3)"
        } else {
            label3.text? = "empty"
        }
        // Do any additional setup after loading the view, typically from a nib.
    }

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

Answer 1

从列表中删除第一个项目后，shoppingList 中将只有 3 个项目，因此访问 shoppingList[3] 将崩溃（记住 3 个项目只有 0..

解决此问题的最简单方法是使用以下模式，以便在使用索引之前检查计数以确保索引有效。

if shoppingList.count > 0 {
    label0.text = shoppingList[0]
} else {
    label0.text = "empty"
}

if shoppingList.count > 1 {
    label1.text = shopingList[1]
} else {
    label1.text = "empty"
}

我还进行了一些额外的更改，例如使用字符串插值将String 转换为相同的String 并非毫无意义，因为[String][n] 将始终返回String（绝不是String? ) 无需处理Optionals

当您尝试以下操作时，您会遇到类似的问题（实际上，可能是您现在遇到的问题）：

shoppingList.removeAtIndex(3)

第二次，因为 3 不再是一个有效的索引，而是使用：

if shoppingList.count > 3 {
    shoppingList.removeAtIndex(3)
}