Math.Log 方法

定义

命名空间:

System

程序集:

System.Runtime.dll

返回指定数字的对数。

重载

Log(Double, Double)	返回指定数字在使用指定底时的对数。
Log(Double)	返回指定数字的自然对数（底为 e）。

Log(Double, Double)

Source:

Math.cs

返回指定数字在使用指定底时的对数。

C#

public static double Log (double a, double newBase);

参数

a

Double

要查找其对数的数字。

newBase

Double

对数的底。

返回

Double

下表中的值之一。 （+Infinity 表示 PositiveInfinity，-Infinity 表示 NegativeInfinity，NaN 表示 NaN。）

a	newBase	返回值
a > 0	(0 <newBase< 1) -或- (newBase> 1)	lognewBase(a)
a < 0	（任意值）	NaN
（任意值）	newBase < 0	NaN
a != 1	newBase = 0	NaN
a != 1	newBase = +Infinity	NaN
a = NaN	（任意值）	NaN
（任意值）	newBase = NaN	NaN
（任意值）	newBase = 1	NaN
a = 0	0 <newBase< 1	+Infinity
a = 0	newBase > 1	-Infinity
a = +Infinity	0 <newBase< 1	-Infinity
a = +Infinity	newBase > 1	+Infinity
a = 1	newBase = 0	0
a = 1	newBase = +Infinity	0

示例

以下示例使用 Log 来计算所选值的某些对数标识。

C#

// Example for the Math.Log( double ) and Math.Log( double, double ) methods.
using System;

class LogDLogDD
{
    public static void Main()
    {
        Console.WriteLine(
            "This example of Math.Log( double ) and " +
            "Math.Log( double, double )\n" +
            "generates the following output.\n" );
        Console.WriteLine(
            "Evaluate these identities with " +
            "selected values for X and B (base):" );
        Console.WriteLine( "   log(B)[X] == 1 / log(X)[B]" );
        Console.WriteLine( "   log(B)[X] == ln[X] / ln[B]" );
        Console.WriteLine( "   log(B)[X] == log(B)[e] * ln[X]" );

        UseBaseAndArg(0.1, 1.2);
        UseBaseAndArg(1.2, 4.9);
        UseBaseAndArg(4.9, 9.9);
        UseBaseAndArg(9.9, 0.1);
    }

    // Evaluate logarithmic identities that are functions of two arguments.
    static void UseBaseAndArg(double argB, double argX)
    {
        // Evaluate log(B)[X] == 1 / log(X)[B].
        Console.WriteLine(
            "\n                   Math.Log({1}, {0}) == {2:E16}" +
            "\n             1.0 / Math.Log({0}, {1}) == {3:E16}",
            argB, argX, Math.Log(argX, argB),
            1.0 / Math.Log(argB, argX) );

        // Evaluate log(B)[X] == ln[X] / ln[B].
        Console.WriteLine(
            "        Math.Log({1}) / Math.Log({0}) == {2:E16}",
            argB, argX, Math.Log(argX) / Math.Log(argB) );

        // Evaluate log(B)[X] == log(B)[e] * ln[X].
        Console.WriteLine(
            "Math.Log(Math.E, {0}) * Math.Log({1}) == {2:E16}",
            argB, argX, Math.Log(Math.E, argB) * Math.Log(argX) );
    }
}

/*
This example of Math.Log( double ) and Math.Log( double, double )
generates the following output.

Evaluate these identities with selected values for X and B (base):
   log(B)[X] == 1 / log(X)[B]
   log(B)[X] == ln[X] / ln[B]
   log(B)[X] == log(B)[e] * ln[X]

                   Math.Log(1.2, 0.1) == -7.9181246047624818E-002
             1.0 / Math.Log(0.1, 1.2) == -7.9181246047624818E-002
        Math.Log(1.2) / Math.Log(0.1) == -7.9181246047624818E-002
Math.Log(Math.E, 0.1) * Math.Log(1.2) == -7.9181246047624804E-002

                   Math.Log(4.9, 1.2) == 8.7166610085093179E+000
             1.0 / Math.Log(1.2, 4.9) == 8.7166610085093161E+000
        Math.Log(4.9) / Math.Log(1.2) == 8.7166610085093179E+000
Math.Log(Math.E, 1.2) * Math.Log(4.9) == 8.7166610085093179E+000

                   Math.Log(9.9, 4.9) == 1.4425396251981288E+000
             1.0 / Math.Log(4.9, 9.9) == 1.4425396251981288E+000
        Math.Log(9.9) / Math.Log(4.9) == 1.4425396251981288E+000
Math.Log(Math.E, 4.9) * Math.Log(9.9) == 1.4425396251981288E+000

                   Math.Log(0.1, 9.9) == -1.0043839404494075E+000
             1.0 / Math.Log(9.9, 0.1) == -1.0043839404494075E+000
        Math.Log(0.1) / Math.Log(9.9) == -1.0043839404494075E+000
Math.Log(Math.E, 9.9) * Math.Log(0.1) == -1.0043839404494077E+000
*/

注解

此方法调用基础 C 运行时，不同的操作系统或体系结构的确切结果或有效输入范围可能有所不同。

Log(Double)

Source:

Math.cs

返回指定数字的自然对数（底为 e）。

C#

public static double Log (double d);

参数

d

Double

要查找其对数的数字。

返回

Double

下表中的值之一。

d 参数	返回值
正	d 的自然对数，即 ln d 或 log e d
零	NegativeInfinity
负数	NaN
等于 NaN	NaN
等于 PositiveInfinity	PositiveInfinity

示例

下面的示例演示 Log 了 方法。

C#

using System;
public class Example
{
   public static void Main()
   {
      Console.WriteLine("  Evaluate this identity with selected values for X:");
      Console.WriteLine("                              ln(x) = 1 / log[X](B)");
      Console.WriteLine();

      double[] XArgs = { 1.2, 4.9, 9.9, 0.1 };

      foreach (double argX in XArgs)
      {
         // Find natural log of argX.
         Console.WriteLine("                      Math.Log({0}) = {1:E16}",
                           argX, Math.Log(argX));

         // Evaluate 1 / log[X](e).
         Console.WriteLine("             1.0 / Math.Log(e, {0}) = {1:E16}",
                           argX, 1.0 / Math.Log(Math.E, argX));
         Console.WriteLine();
      }
   }
}
// This example displays the following output:
//         Evaluate this identity with selected values for X:
//                                     ln(x) = 1 / log[X](B)
//
//                             Math.Log(1.2) = 1.8232155679395459E-001
//                    1.0 / Math.Log(e, 1.2) = 1.8232155679395459E-001
//
//                             Math.Log(4.9) = 1.5892352051165810E+000
//                    1.0 / Math.Log(e, 4.9) = 1.5892352051165810E+000
//
//                             Math.Log(9.9) = 2.2925347571405443E+000
//                    1.0 / Math.Log(e, 9.9) = 2.2925347571405443E+000
//
//                             Math.Log(0.1) = -2.3025850929940455E+000
//                    1.0 / Math.Log(e, 0.1) = -2.3025850929940455E+000

注解

参数 d 指定为基数 10。

此方法调用基础 C 运行时，不同的操作系统或体系结构的确切结果或有效输入范围可能有所不同。

此方法调用基础 C 运行时，不同的操作系统或体系结构的确切结果或有效输入范围可能有所不同。

另请参阅

• E
• Exp(Double)