Question

我想通过startYear搜索特定的美国总统，我试图在Swift 4中使用过滤器。我无法让它工作。我故意使用Codable协议。

let x = {
  test1: 4,
  test2: 5,
  'test3': 6
};

x.test1 // is 4
x.test2 // is 5
x.test3 // is 6

x['test1'] // is 4
x['test2'] // is 5
x['test3'] // is 6

const y = 'test1';

x.y // undefined.  we never set a key of y.

x[y] // this is interpreted as x['test1'] so the result is 4

Answer 1

使用年份范围而不是单独的Ints，然后您可以执行包含检查：

struct USPresident: Codable {
    let firstName: String
    let lastName: String
    let startYear: Int
    let endYear: Int

    var yearsInOffice: CountableClosedRange<Int> {
        return startYear...endYear
    }
}

let allPresidents = [
    USPresident(firstName: "Widrow",    lastName: "Wilson",         startYear: 1913, endYear: 1921),
    USPresident(firstName: "Frankly",   lastName: "Roosevelt",      startYear: 1933, endYear: 1945),
    USPresident(firstName: "John",      lastName: "Kennedy",        startYear: 1961, endYear: 1963),
    USPresident(firstName: "Dwight",    lastName: "Eisenhowser",    startYear: 1953, endYear: 1961),
    USPresident(firstName: "Andrew",    lastName: "Jackson",        startYear: 1865, endYear: 1869),
    USPresident(firstName: "George",    lastName: "Washington",     startYear: 1789, endYear: 1796),
]

let desiredYears = [1953]
let result = allPresidents.filter { president in
    desiredYears.contains(where:) { president.yearsInOffice.contains($0) }
}
print("\n\nPresident in 1953 was:  = \(result)\n\n")

Answer 2

您希望过滤器查看searchFor是否包含startYear。

let result = allPresidents.filter { searchFor.contains($0.startYear) }

Answer 3

请注意，Codable几乎没有为您的问题增加任何值，因为您的查询在已经解码的对象上运行，Codable有助于从原始数据到结构实例的转换。

此外，将searchFor作为数组只会使某些事情变得复杂，如果你想知道在某一年中谁是总统。我建议回到简单的一年。

现在，进入追逐，您可以使用first(where:)方法从列表中找出与给定查询匹配的第一个元素。

let searchFor = 1953
let result = allPresidents.first(where: { $0.startYear...$0.endYear ~= searchFor })
print("\n\n President in \(searchFor) was \(result)\n\n")

只需注意first(where:)返回一个可选项，因此如果要将值向前传递，则可能需要解包该值。

iOS Struct Codable和过滤器？

3 个答案: