I have an enumeration MyEnum (N开发者_如何学Pythoneg -1; None 0; Pos 1).

I use the result of CompareTo() to initialize a object of that enum.

What is the most performant way in .NET to do it (Negative to -1, 0 to 0, Positive to 1)?

NB.

CompareTo() returns an Integer value..

Have a look at Math.Sign

there's a builtin method called Math.Sign in the .net framework

As @Henrik and @knittl have said you should use Math.Sign. But if you're interested as to what the .Net framework is doing behind the scenes the following code has come out of Reflector.

public static int Sign(int value)
{
  if (value < 0)
  {
    return -1;
  }
  if (value > 0)
  {
    return 1;
  }
  return 0;
}

Math.Sign(value)

Result of CompareTo is negative, zero, or positive. If you look at the other answers, Math.Sign uses 2 if statements to return an int.

Just re-code Math.Sign to return your enum.

(If this was not homework I would give a code example there, but you are supposed to learn from it).

After that, test it, to see what is the most performant.

Test Results (Dual core, x86):

''''''''''''''''''''' DEBUG MODE '''
= 1 =
Division took    00:00:06.2482408 ms
BuiltInSign took 00:00:05.0293383 ms <<<
BitTestSign took 00:00:05.2092181 ms
CustomSign took  00:00:05.2512802 ms

= 2 =
Division took    00:00:06.2477787 ms
BuiltInSign took 00:00:05.0330921 ms <<<
BitTestSign took 00:00:05.2114098 ms
CustomSign took  00:00:05.2556966 ms

= 3 =
Division took    00:00:06.2506690 ms
BuiltInSign took 00:00:05.0388615 ms <<<
BitTestSign took 00:00:05.2306954 ms
CustomSign took  00:00:05.2512391 ms


''''''''''''''''''' RELEASE MODE '''
= 1 =
Division took    00:00:01.0974078 ms
BuiltInSign took 00:00:00.3195232 ms
BitTestSign took 00:00:00.6392142 ms
CustomSign took  00:00:00.3194230 ms <<<

= 2 =
Division took    00:00:01.1007138 ms
BuiltInSign took 00:00:00.3197784 ms <<<
BitTestSign took 00:00:00.6395294 ms
CustomSign took  00:00:00.3202774 ms

= 3 =
Division took    00:00:01.0977087 ms
BuiltInSign took 00:00:00.3194622 ms <<<
BitTestSign took 00:00:00.6394220 ms
CustomSign took  00:00:00.3201607 ms

Code:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        Stopwatch sw = new Stopwatch();
        MyEnum myEnum = MyEnum.None;

        const int max = 100000000;

        sw.Start();
        for (int i = -max; i < max; i++)
        {
            myEnum = Division(i);
        }
        sw.Stop();
        Console.WriteLine("Division took {0} ms", sw.Elapsed);
        sw.Reset();

        sw.Start();
        for (int i = -max; i < max; i++)
        {
            myEnum = BuiltInSign(i);
        }
        sw.Stop();
        Console.WriteLine("BuiltInSign took {0} ms", sw.Elapsed);
        sw.Reset();

        sw.Start();
        for (int i = -max; i < max; i++)
        {
            myEnum = BitTestSign(i);
        }
        sw.Stop();
        Console.WriteLine("BitTestSign took {0} ms", sw.Elapsed);
        sw.Reset();

        sw.Start();
        for (int i = -max; i < max; i++)
        {
            myEnum = CustomSign(i);
        }
        sw.Stop();
        Console.WriteLine("CustomSign took {0} ms", sw.Elapsed);
    }

    private MyEnum Division(int value)
    {
        return value == 0 ? 0 : (MyEnum)(value / Math.Abs(value));
    }

    private MyEnum BuiltInSign(int value)
    {
        return (MyEnum)Math.Sign(value);
    }

    private MyEnum CustomSign(int value)
    {
        if (value < 0)
            return MyEnum.Neg;

        if (value > 0)
            return MyEnum.Pos;

        return MyEnum.None;
    }

    MyEnum BitTestSign(int value)
    {
        // Shifts the variable over 31 places, 
        // if the MSB is 1, the statement is true
        if ((value >> 31) == 1)
        {
            return MyEnum.Neg;
        }
        else
        {
            if (value == 0)
            {
                return MyEnum.None;
            }
            else
            {
                return MyEnum.Pos;
            }
        }
    }

    private enum MyEnum
    {
        Pos = 1,
        None = 0,
        Neg = -1
    }
}

.NET internally stores ints as two's complement. So, if you want to try something else, check and see if the most significant bit is set. This may or may not be faster, but it should fit into your testing framework easily.

Pseudocode:

if(num == 0)
  return 0;

if(num has msb set)
  return -1;

return 1;

If I understand that correctly, you want to return -1 for negative values, 0 for zero values and 1 for positive values.

I'd go with:

public static MyEnum GetSign(int value)
{
    return value == 0 ? 0 : (MyEnum)(value / Math.Abs(value));
}

Or did I get something wrong?

You could check the most significant bit of the variable (in two's complement, if the most significant bit is 1, you have a negative number). If your most significant bit is 0, check if the value is 0. Return a value accordingly.

int Sign(int value){
    if (value >> 31) { //Shifts the variable over 31 places, if the MSB is 1, the statement is true
         return -1;
    } else {
        if (value == 0){
            return 0;
        } else {
            return 1;
        }
    }  }  

(edited for example)