swift - 表达式类型 '()' 在没有更多 XCTAssertEqual 上下文的情况下不明确

Question

当我调用测试方法 XCTAssertEqual 时，我看到错误 Expression type '()' is ambiguous without more context。我已经尝试将 [(Character,Int)] 明确添加到 expected 但我仍然看到此错误。我不确定还能尝试什么。

如何让 swift 编译器理解？

测试:

func testFrequencyCountOnInput() {
    let huff = Huffman("ANNA")
    let expected: [(Character,Int)] = [("A", 2), ("N", 2)]
    XCTAssertEqual(huff.frequency, expected)
}

代码:

class Huffman {
    private(set) var frequency: [(Character, Int)]

    init(_ input: String) {
        self.frequency = Huffman.sortedFrequency(for: input)
    }

    static private func sortedFrequency(for input: String) -> [(Character, Int)] {
        var frequency = [Character: Int]()
        for char in input {
            frequency[char] = (frequency[char] ?? 0) + 1
        }
        let sorted = Array(frequency).sorted(by: {$0.1 > $1.1})
        return sorted
    }
}

Answer 1

我认为在这里使用 Tuple 不是一个好主意。相反，您可以将其重构为某种类型。例如:

struct Frequency: Equatable {
    let char: Character
    let count: Int
}

Struct 很好，它是一种值类型(作为 Tuple)并且 can automatically synthesize conformance to Equatable protocol .

下一步是 Huffman 类。对于这样一个小问题，你有大量的代码。让我们也尝试重构它:

final class Huffman {
    private(set) var frequency: [Frequency]

    init(_ input: String) {
        frequency = input.counts
            .map { Frequency(char: $0.key, count: $0.value) }
            .sorted(by: { $0.count > $1.count })
    }
}

extension String {
    var counts: [Character: Int] {
        return reduce(into: [:]) { d, c in
            d[c] = d[c, default: 0] + 1
        }
    }
}

请注意，此代码计算String 扩展的counts 属性(可能不是最好的名称)中的频率，然后将结果map 到[Frequency] 最后，排序也不是什么大问题。代码更少，更准确。
在这里我们进行单一测试:

func testFrequencyCountOnInput() {
    let huff = Huffman("ANNNA")
    let expected: [Frequency] = [
        Frequency(char: "N", count: 3),
        Frequency(char: "A", count: 2)
    ]
    XCTAssertEqual(huff.frequency, expected)
}

一切正常，甚至是 sort 功能。