Swift 中不区分大小写的字典

Question

给定一个Dictionary，其Key 的类型为String，有没有办法以不区分大小写的方式访问该值？例如：

let dict = [
    "name": "John",
    "location": "Chicago"
]

有没有办法打电话dict["NAME"], dict["nAmE"]等，仍然得到"John"？

Answer 1

更简洁的方法，swift 4：

extension Dictionary where Key == String {

    subscript(caseInsensitive key: Key) -> Value? {
        get {
            if let k = keys.first(where: { $0.caseInsensitiveCompare(key) == .orderedSame }) {
                return self[k]
            }
            return nil
        }
        set {
            if let k = keys.first(where: { $0.caseInsensitiveCompare(key) == .orderedSame }) {
                self[k] = newValue
            } else {
                self[key] = newValue
            }
        }
    }

}

// Usage:
var dict = ["name": "John"]
dict[caseInsensitive: "NAME"] = "David" // overwrites "name" value
print(dict[caseInsensitive: "name"]!) // outputs "David"

Answer 2

Swift 支持多个下标，因此您可以利用它来定义不区分大小写的访问器：

extension Dictionary where Key : StringLiteralConvertible {
    subscript(ci key : Key) -> Value? {
        get {
            let searchKey = String(key).lowercaseString
            for k in self.keys {
                let lowerK = String(k).lowercaseString
                if searchKey == lowerK {
                    return self[k]
                }
            }
            return nil
        }
    }
}

// Usage:
let dict = [
    "name": "John",
    "location": "Chicago",
]

print(dict[ci: "NAME"])      // John
print(dict[ci: "lOcAtIoN"])  // Chicago

此扩展仅限于Dictionary，其Key 的类型为String（因为小写对其他数据类型没有意义）。但是，Swift 会抱怨将泛型类型限制为struct。最接近String 的协议是StringLiteralConvertible。

请注意，如果您有 2 个小写形式相同的键，则无法保证您会返回哪一个：

let dict = [
    "name": "John",
    "NAME": "David",
]

print(dict[ci: "name"])   // no guarantee that you will get David or John.

Answer 3

现有的答案很好，但是使用这些策略进行查找/插入的时间复杂度从 O(1) 恶化到 O(N)（其中 N 是字典中的对象数）。

要保留 O(1)，您可能需要考虑以下方法：

/// Wrapper around String which uses case-insensitive implementations for Hashable
public struct CaseInsensitiveString: Hashable, LosslessStringConvertible, ExpressibleByStringLiteral {
    public typealias StringLiteralType = String

    private let value: String
    private let caseInsensitiveValue: String

    public init(stringLiteral: String) {
        self.value = stringLiteral
        self.caseInsensitiveValue = stringLiteral.lowercased()
    }

    public init?(_ description: String) {
        self.init(stringLiteral: description)
    }

    public var hashValue: Int {
        return self.caseInsensitiveValue.hashValue
    }

    public static func == (lhs: CaseInsensitiveString, rhs: CaseInsensitiveString) -> Bool {
        return lhs.caseInsensitiveValue == rhs.caseInsensitiveValue
    }

    public var description: String {
        return value
    }
}

var dict = [CaseInsensitiveString: String]()
dict["name"] = "John"
dict["NAME"] = "David" // overwrites "name" value
print(dict["name"]!) // outputs "David"

Answer 4

可以使用Collection's first(where:)从所有映射为小写的键中找到第一个小写匹配，然后从这个结果中返回值。

extension Dictionary where Key == String {
    func valueForKeyInsensitive<T>(key: Key) -> T? {
        let foundKey = self.keys.first { $0.compare(key, options: .caseInsensitive) == .orderedSame } ?? key
        return self[foundKey] as? T
    }
}

first(where:) 是过滤或迭代大型集合的一种非常有效的方法

参考：

• https://developer.apple.com/documentation/swift/anybidirectionalcollection/2906322-first#

• https://github.com/realm/SwiftLint/blob/master/Rules.md#first-where

Answer 5

这应该使用 O(1) 完成这项工作，同时也不允许添加具有不同大小写的相同字符串（例如，如果您首先插入 Def 它不会被 DEF 替换）。如有必要，它也适用于 Substring。请注意，此解决方案更节省内存，但代价是在每次查找字符串时重新计算字符串转换和散列。如果您需要经常查找相同的值，则可能值得拥有一个缓存 hashValue 的实现。

struct CaseInsensitiveString<T: StringProtocol>: Hashable, Equatable, CustomStringConvertible {
    var string: T

    init(_ string: T) {
        self.string = string
    }

    var description: String { get {
        return string.description
    }}

    var hashValue: Int { get {
        string.lowercased().hashValue
    } }

    func hash(into hasher: inout Hasher) {
        hasher.combine(hashValue)
    }

    static func == (lhs: Self, rhs: Self) -> Bool {
        return lhs.string.compare(rhs.string, options: .caseInsensitive) == .orderedSame
    }
}

typealias SubstringCI = CaseInsensitiveString<String>

var codeMap = [SubstringCI: Int]()
let test = "Abc Def Ghi"
let testsub = test[test.firstIndex(of: "D")!...test.lastIndex(of: "f")!]
codeMap[SubstringCI(String(testsub))] = 1
print(codeMap.keys, codeMap[SubstringCI("Def")]!, codeMap[SubstringCI("def")]!)
codeMap[SubstringCI("DEF")] = 1
print(codeMap.keys, codeMap[SubstringCI("Def")]!, codeMap[SubstringCI("def")]!)