swift - 性能：Array.removeAll vs `= []`

Question

ArraySwift /Dictionary原始函数之间在计算removeAll和/或内存方面的性能是否存在差异init？基本上，我在问，在 Swift 中重置可变集合的优缺点是什么，是否被认为是清空Array/的推荐方法Dictionary？

// Scenario A
var myArray = ["one", "two", "three"]
myArray.removeAll()

// Scenario B
myArray = ["one", "two", "three"]
myArray = []

标签： swiftperformancememorycollections

Answer 1

性能差异不应该是显着的，因为它们在很大程度上做同样的事情。让我们看一下源代码：

/// Removes all elements from the array.
///
/// - Parameter keepCapacity: Pass `true` to keep the existing capacity of
///   the array after removing its elements. The default value is
///   `false`.
///
/// - Complexity: O(*n*), where *n* is the length of the array.
@inlinable
public mutating func removeAll(keepingCapacity keepCapacity: Bool = false) {
  if !keepCapacity {
    _buffer = _Buffer()
  }
  else {
    self.replaceSubrange(indices, with: EmptyCollection())
  }
}

对于不同的类型，此方法还有其他几种实现，但模式始终相同。如果removeAll参数keepCapacity是false，只需重新初始化它，很大程度上相当于说myArray = []。

因此，唯一的问题是您是否要在删除其元素后保留数组的容量（如果您要清空一个大数组并准备用另一个相同大小的数组重新填充，您可能会这样做）。

---

如果您愿意，请对其进行基准测试。例如，将“单元测试”目标添加到您的项目中，将迭代次数提高到足以使持续时间可观察到：

class MyAppTests: XCTestCase {

    func testPerformanceRemoveAll() {
        var countTotal = 0
        var myArray: [Int] = []

        self.measure {
            for _ in 0 ..< 1_000_000 {
                myArray = Array(repeating: 0, count: 1_000)
                myArray.removeAll(keepingCapacity: false)
                countTotal += myArray.count
            }
        }

        XCTAssertEqual(countTotal, 0)
    }

    func testPerformanceReinitialize() {
        var countTotal = 0
        var myArray: [Int] = []

        self.measure {
            for _ in 0 ..< 1_000_000 {
                myArray = Array(repeating: 0, count: 1_000)
                myArray = []
                countTotal += myArray.count
            }
        }

        XCTAssertEqual(countTotal, 0)
    }

}

结果如下：

Test Case '-[MyAppTests.MyAppTests testPerformanceReinitialize]' started.
/.../MyApp/MyAppTests/MyAppTests.swift:41: Test Case '-[MyAppTests.MyAppTests testPerformanceReinitialize]' measured [Time, seconds] average: 0.221, relative standard deviation: 6.559%, values: [0.264467, 0.216076, 0.216146, 0.216040, 0.216014, 0.216426, 0.216374, 0.215876, 0.216272, 0.216152], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[MyAppTests.MyAppTests testPerformanceReinitialize]' passed (2.646 seconds).

Test Case '-[MyAppTests.MyAppTests testPerformanceRemoveAll]' started.
/.../MyApp/MyAppTests/MyAppTests.swift:26: Test Case '-[MyAppTests.MyAppTests testPerformanceRemoveAll]' measured [Time, seconds] average: 0.235, relative standard deviation: 6.712%, values: [0.282223, 0.229732, 0.229601, 0.229624, 0.229584, 0.229652, 0.229695, 0.229729, 0.229702, 0.229659], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[MyAppTests.MyAppTests testPerformanceRemoveAll]' passed (2.602 seconds).

顺便说一句，如果您想知道为什么我在清空数组后添加总数，我只是想确保在清空数组后我实际使用了数组，以确保优化器不会优化进行清空的代码。在这种情况下没有必要，但要谨慎。

我也用Int代替 进行了测试String，因为我对String开销并不好奇，而是试图专注于Array行为。

最重要的是，性能差异在很大程度上无法区分。