Swift中不区分大小写的字典

Question

鉴于Dictionary的Key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"
]

有没有办法打电话给qazxsw poi等，仍然可以获得qazxsw poi？

Answer 1

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者：

dict["NAME"], dict["nAmE"]

此扩展仅限于"John"，其

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

的类型为Dictionary（因为小写与其他数据类型无关）。但是，Swift会抱怨将一般类型限制为Key。最接近String的协议是struct。

请注意，如果您有两个小写形式相同的键，则无法保证您将获得哪一个：

String

Answer 2

更清洁的方法，迅捷4：

StringLiteralConvertible

Answer 3

现有的答案很好，但是那些策略的查找/插入的时间复杂度从O（1）到O（N）（其中N是字典中的对象数）恶化。

要保留O（1），您可能需要考虑以下方法：

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

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

Answer 4

可以使用

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"

从映射为小写的所有键中找到第一个小写匹配，然后从此结果返回值。

/// 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"

Collection's first(where:)是一种过滤或迭代大型集合的高效方法

参考：

• 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:)