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Swift 5:为什么会出现索引超出范围错误?

indexoutofrangeexception swift arrays

maxMas · 技术社区 · 2 年前

我有两个数组,我将包括在一个带有对象实例的数组中。当我尝试这样做时,我得到一个错误,索引超出范围。

我的代码如下。我不明白为什么会出现这个错误。有人能告诉我我做错了什么吗?

import UIKit

let mealName = ["Breakfast", "Lunch", "Dinner", "Breakfast", "Lunch", "Dinner", "Breakfast", "Lunch", "Dinner", "Snack", "Supper", "Snack", "Breakfast", "Snack", "Lunch", "Dinner", "Supper", "Snack", "Breakfast", "Snack", "Dinner", "Snack", "Snack", "Snack", "Breakfast", "Snack", "Lunch", "Snack", "Snack", "Snack", "Breakfast", "Snack", "Lunch", "Snack", "Snack", "Snack", "Snack"]

let mealCalories = [725.03, 539.44, 849.49, 956.53, 267.18, 567.17, 353.47, 387.12, 700.76, 62.87, 857.35, 176.56, 123.79, 16.74, 813.74, 858.23, 127.26, 448.19, 932.62, 60.22, 880.55, 58.85, 78.48, 21.5, 564.74, 190.75, 911.01, 67.47, 81.44, 25.35, 954.94, 45.75, 858.71, 61.7, 189.75, 72.54, 75.84]


struct Meal {
    var mealName: String
    var mealCalories: Double
}

var meals = [Meal]()

for i in 0..<37 {
    meals[i].mealName = mealName[i]
    meals[i].mealCalories = mealCalories[i]
}

1 回复 | 直到 2 年前

Asteroid 2 年前

var meals: [Meal] = [Meal](repeating: Meal(mealName: "", mealCalories: 0.0), count: 37)

然后您可以参考膳食[i](0<=i<37)。

如果要从空数组开始,可以向其中添加元素:

var meals = [Meal]()

然后:

for i in 0..<37 {
    let meal = Meal(mealName: mealName[i], mealCalories: mealCalories[i])
    meals.append(meal)
}

在这种情况下,食物的长度会随着我的成长而增加。

Alexander 2 年前

你已经有一个 Meal 这就引出了一个问题:为什么不直接定义你的饮食?

struct Meal {
    // Don't prefix the properties with "meal".
    // It leads to pointless code like "someMeal.mealName".
    // You already know it's a meal by the type.
    let name: String
    let calories: Double
}

let meals = [
    let meals = [
    Meal(name: "Breakfast", calories: 725.03),
    Meal(name: "Lunch",     calories: 539.44),
    Meal(name: "Dinner",    calories: 849.49),
    Meal(name: "Breakfast", calories: 956.53),
    Meal(name: "Lunch",     calories: 267.18),
    Meal(name: "Dinner",    calories: 567.17),
    Meal(name: "Breakfast", calories: 353.47),
    Meal(name: "Lunch",     calories: 387.12),
    Meal(name: "Dinner",    calories: 700.76),
    Meal(name: "Snack",     calories:  62.87),
    Meal(name: "Supper",    calories: 857.35),
    Meal(name: "Snack",     calories: 176.56),
    Meal(name: "Breakfast", calories: 123.79),
    Meal(name: "Snack",     calories:  16.74),
    Meal(name: "Lunch",     calories: 813.74),
    Meal(name: "Dinner",    calories: 858.23),
    Meal(name: "Supper",    calories: 127.26),
    Meal(name: "Snack",     calories: 448.19),
    Meal(name: "Breakfast", calories: 932.62),
    Meal(name: "Snack",     calories:  60.22),
    Meal(name: "Dinner",    calories: 880.55),
    Meal(name: "Snack",     calories:  58.85),
    Meal(name: "Snack",     calories:  78.48),
    Meal(name: "Snack",     calories:  21.50),
    Meal(name: "Breakfast", calories: 564.74),
    Meal(name: "Snack",     calories: 190.75),
    Meal(name: "Lunch",     calories: 911.01),
    Meal(name: "Snack",     calories:  67.47),
    Meal(name: "Snack",     calories:  81.44),
    Meal(name: "Snack",     calories:  25.35),
    Meal(name: "Breakfast", calories: 954.94),
    Meal(name: "Snack",     calories:  45.75),
    Meal(name: "Lunch",     calories: 858.71),
    Meal(name: "Snack",     calories:  61.70),
    Meal(name: "Snack",     calories: 189.75),
    Meal(name: "Snack",     calories:  72.54),
    Meal(name: "Snack",     calories:  75.84),
]

如果并行阵列必须保留

(例如,它们来自您无法控制的API)

一个数组已经存储了它自己的计数,而在其他地方计数的硬编码引入了分裂的真值源。这里的风险是它们不同步:
- 如果添加了新的餐名/卡路里,但您忘记更新 37
- 如果删除了一些餐名/卡路里,那么您的将过大并导致越界错误。
您将索引到一个空数组中,而不是追加

幸运的是,这些问题很容易解决!

将索引修复到末尾,并使用 append 相反:

var meals = [Meal]()
for i in 0..<37 {
    meals.append(Meal(name: mealNames[i], calories: mealCalories[i])
}

assert(mealNames.count == mealCalories.count,
    "Config error! The names and calories should match up one for one!")

var meals = [Meal]()

for i in 0..<meanNames.count {
    meals.append(Meal(name: mealNames[i], calories: mealCalories[i])
}

assert(mealNames.count == mealCalories.count, ...)

var meals = [Meal]()

for i in mealNames.indices {
    meals.append(Meal(name: mealNames[i], calories: mealCalories[i])
}

完全忘记索引!只需使用 zip(_:_:) :

assert(mealNames.count == mealCalories.count, ...)

var meals = [Meal]()

for (name, calories) in zip(mealNames, mealCalories) {
    meals.append(Meal(name: name, calories: calories))
}

迭代输入、转换输入并生成新数组只是一个映射操作。

assert(mealNames.count == mealCalories.count, ...)

let meals = zip(mealNames, mealCalories).map { name, calories in
    Meal(name: name, calories: calories))
}

meals 不是通过重复突变构建的,我们现在可以使其成为 let 常数