println 字典有“可选" [英] println dictionary has "Optional"

问题描述

考虑这个片段:

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
println(interestingNumbers["Square"])

let individualScores = [75, 43, 103, 87, 12]
println(individualScores)

控制台输出:

可选([1, 4, 9, 16, 25])

Optional([1, 4, 9, 16, 25])

[75, 43, 103, 87, 12]

[75, 43, 103, 87, 12]

为什么词典案例中有一个可选"?

Why was there an "Optional" in the dictionary case?

推荐答案

Swift 词典正在返回选项以确保安全.如果您尝试访问一个不存在的密钥，那会给您零.

Swift dictionaries are returning optionals for safety. If you try to access a key which does not exists that would give you nil.

您还可以使用下标语法从字典中检索特定键的值.因为可以请求不存在值的键，所以字典的下标返回字典值类型的可选值.如果字典包含请求键的值，则下标返回一个可选值，其中包含该键的现有值.否则下标返回nil

You can also use subscript syntax to retrieve a value from the dictionary for a particular key. Because it is possible to request a key for which no value exists, a dictionary’s subscript returns an optional value of the dictionary’s value type. If the dictionary contains a value for the requested key, the subscript returns an optional value containing the existing value for that key. Otherwise, the subscript returns nil

来自 Swift 编程语言

和

使用下标访问任何字典中的单个元素.从字典的下标返回的值是 ValueType 类型吗?— 具有字典 ValueType 基础类型的可选项

Use subscripting to access the individual elements in any dictionary. The value returned from a dictionary's subscript is of type ValueType? — an optional with an underlying type of the dictionary’s ValueType

来自 Swift 标准库参考

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