-1

几天来，我一直在努力让这段代码更快、更高效，但它似乎仍然没有达到应有的效率。

Sub Main()
    Dim watch As Stopwatch = Stopwatch.StartNew()

    Dim l As New List(Of ULong)(CType(My.Application.CommandLineArgs.Item(0), ULong))
    For i As ULong = 1 To l.Capacity
        ' ONE LINE IF STMT: If l.Capacity And 1 <> 0 Then If i And 1 = 0 Then l.Add((i * i) - 1) Else l.Add((i * i) + 1) Else If i And 1 = 0 Then l.Add((i * i) + 1) Else l.Add((i * i) - 1)
        If l.Capacity And 1 <> 0 Then
            If i And 1 = 0 Then
                l.Add((i * i) - 1)
            Else
                l.Add((i * i) + 1)
            End If
        Else
            If i And 1 = 0 Then
                l.Add((i * i) + 1)
            Else
                l.Add((i * i) - 1)
            End If
        End If
    Next i
    Console.WriteLine(String.Join(","c, l.ToArray))

    watch.Stop()
    Console.WriteLine(watch.Elapsed.TotalMilliseconds)
    Console.ReadLine()
End Sub

它目前在 4.3 毫秒内运行 100 次 i 迭代。我觉得嵌套的 if 语句是这里的主要瓶颈，但我不确定有什么方法可以改变它们。

那么，这段代码可以变得更高效吗？

谢谢... ：）

4

2 回答 2

1

首先，If l.Capacity And 1 <> 0退出循环，因为它总是一样的。

然后，放弃位操作以支持更具可读性的i Mod 2 = 0测试：您似乎认为这更有效，但实际上这种微不足道的优化最好留给编译器和运行时，它们在您的代码中没有位置，我严重怀疑他们有任何可衡量的影响。

如果这仍然效率低下，请不要使用初始容量和Add. 相反，使用Resize和索引访问元素。然后，您还可以通过使用两个循环来摆脱其他If语句：一个用于偶数元素，一个用于奇数元素。

也就是说，该Join操作可能是迄今为止最慢的步骤（除了实际打印到控制台之外，也许）并且您无法优化它。

最后，我发现CType非常难以理解：你在这里所做的不是强制转换，而是解析操作。为什么不把它写成一个呢？ULong.Parse(…). 另外，您为什么使用笨拙的My.Application.CommandLineArgs.Item而不是接受命令行参数作为参数Main？

于 2012-07-28T12:01:49.130 回答
0

一旦我只更正了您的错误，正如我认为您所希望的那样，当使用 Ctrl+F5 和带有发布版本的 Visual Studio 2010 时，我会得到以下输出：

0,5,8,17,24,37,48,65,80,101,120,145,168,197,224,257,288,325,360,401,440,485,528,577,624,677,728,785,840,901,960,1025,1088,1157,1224,1297,1368,1445,1520,1601,1680,1765,1848,1937,2024,2117,2208, 2305,2400,2501,2600,2705,2808,2917,3024,3137,3248,3365,3480,3601,3720,3845,3968,4097,4224,4357,4488,4625,4760,4901,5040,5185, 5328,5477,5624,5777,5928,6085,6240,6401,6560,6725,6888,7057,7224,7397,7568,7745,7920,8101,8280,8465,8648,8837,9024,9217,9408, 9605,9800,10001
33.9106

这就是这段代码：

Sub Main()
    Dim watch As Stopwatch = Stopwatch.StartNew()

    Dim l As New List(Of ULong)(CInt(My.Application.CommandLineArgs(0)))

    For i As ULong = 1 To CULng(l.Capacity)
        ' ONE LINE IF STMT: If l.Capacity And 1 <> 0 Then If i And 1 = 0 Then l.Add((i * i) - 1) Else l.Add((i * i) + 1) Else If i And 1 = 0 Then l.Add((i * i) + 1) Else l.Add((i * i) - 1)
        If (l.Capacity And 1) <> 0 Then
            If (i And 1UL) = 0UL Then
                l.Add((i * i) - 1UL)
            Else
                l.Add((i * i) + 1UL)
            End If
        Else
            If (i And 1UL) = 0UL Then
                l.Add((i * i) + 1UL)
            Else
                l.Add((i * i) - 1UL)
            End If
        End If
    Next i
    Console.WriteLine(String.Join(","c, l.ToArray))

    watch.Stop()
    Console.WriteLine(watch.Elapsed.TotalMilliseconds)
    Console.ReadLine()
End Sub

使用 LINQ：

0,5,8,17,24,37,48,65,80,101,120,145,168,197,224,257,288,325,360,401,440,485,528,577,624,677,728,785,840,901,960,1025,1088,1157,1224,1297,1368,1445,1520,1601,1680,1765,1848,1937,2024,2117,2208, 2305,2400,2501,2600,2705,2808,2917,3024,3137,3248,3365,3480,3601,3720,3845,3968,4097,4224,4357,4488,4625,4760,4901,5040,5185, 5328,5477,5624,5777,5928,6085,6240,6401,6560,6725,6888,7057,7224,7397,7568,7745,7920,8101,8280,8465,8648,8837,9024,9217,9408, 9605,9800,10001
13.0721

那是：

Sub Main()
    Dim watch As Stopwatch = Stopwatch.StartNew()

    Dim arg = CInt(My.Application.CommandLineArgs(0))
    Console.WriteLine(String.Join(","c, Enumerable.Range(1, arg).Select(
                                  Function(i) ((i * i +
                                               If((arg And 1) <> 0,
                                                   If((i And 1) = 0, -1, 1),
                                                   If((i And 1) = 0, 1, -1))).ToString()))))

    watch.Stop()
    Console.WriteLine(watch.Elapsed.TotalMilliseconds)
    Console.ReadLine()
End Sub

现在优化：

0,5,8,17,24,37,48,65,80,101,120,145,168,197,224,257,288,325,360,401,440,485,528,577,624,677,728,785,840,901,960,1025,1088,1157,1224,1297,1368,1445,1520,1601,1680,1765,1848,1937,2024,2117,2208, 2305,2400,2501,2600,2705,2808,2917,3024,3137,3248,3365,3480,3601,3720,3845,3968,4097,4224,4357,4488,4625,4760,4901,5040,5185, 5328,5477,5624,5777,5928,6085,6240,6401,6560,6725,6888,7057,7224,7397,7568,7745,7920,8101,8280,8465,8648,8837,9024,9217,9408, 9605,9800,10001
12.5956

在这一点上，我认为我清楚地达到了Console.WriteLine时间范围内的差异，但即使使用下面的代码，我仍然得到 12 到 15 毫秒......

Sub Main()
    Dim watch As Stopwatch = Stopwatch.StartNew()

    Dim arg = CInt(My.Application.CommandLineArgs(0))

    Dim fn As Func(Of Integer, String) = If((arg And 1) <> 0,
                                            Function(i As Integer) ((i * i + If((i And 1) = 0, -1, 1)).ToString()),
                                            Function(i As Integer) ((i * i + If((i And 1) = 0, 1, -1)).ToString()))

    Dim s = String.Join(","c, Enumerable.Range(1, arg).Select(fn))

    watch.Stop()
    Console.WriteLine(s)
    Console.WriteLine(watch.Elapsed.TotalMilliseconds)
    Console.ReadLine()
End Sub
于 2012-07-30T08:32:34.200 回答