C# | Math.Log() Method

In C#, Math.Log() is a Math class method. It is used to return the logarithm of a specified number. This method can be overloaded by changing the number of the arguments passed. There are total 2 methods in the overload list of the Math.Log() method as follows:

• Math.Log(Double) Method
• Math.Log(Double, Double) Method

Math.Log(Double) Method

This method is used to return the natural (base e) logarithm of a specified number.

Syntax:

public static double Log(double val)

Parameter:

val: It is the specified number whose natural (base e) logarithm to be calculated and its type is System.Double.

Return Value: Returns the natural logarithm of val and its type is System.Double.

Note: Parameter val is always specified as a base 10 number.The return value is depend on the argument passed. Below are some cases:

• If the argument is positive then method will return the natural logarithm or log_e(val).
• If the argument is zero, then the result is NegativeInfinity.
• If the argument is Negative(less than zero) or equal to NaN, then the result is NaN.
• If the argument is PositiveInfinity, then the result is PositiveInfinity.
• If the argument is NegativeInfinity, then the result is NaN.

Example:

// C# program to demonstrate working
// of Math.Log(Double) method
using System;

class Geeks {

    // Main Method
    public static void Main(String[] args)
    {

        // double values whose logarithm
        // to be calculated
        double a = 4.55;
        double b = 0;
        double c = -2.45;
        double nan = Double.NaN;
        double positiveInfinity = Double.PositiveInfinity;
        double negativeInfinity = Double.NegativeInfinity;

        // Input is positive number so output
        // will be logarithm of number
        Console.WriteLine(Math.Log(a));

        // positive zero as argument, so output
        // will be -Infinity
        Console.WriteLine(Math.Log(b));

        // Input is negative number so output
        // will be NaN
        Console.WriteLine(Math.Log(c));

        // Input is NaN so output
        // will be NaN
        Console.WriteLine(Math.Log(nan));

        // Input is PositiveInfinity so output
        // will be Infinity
        Console.WriteLine(Math.Log(positiveInfinity));

        // Input is NegativeInfinity so output
        // will be NaN
        Console.WriteLine(Math.Log(negativeInfinity));
    }
}

Output:

1.51512723296286
-Infinity
NaN
NaN
Infinity
NaN

Math.Log(Double, Double) Method

This method is used to return the logarithm of a specified number in a specified base.

Syntax:

public static double Log(double val, double base)

Parameter:

val: It is the specified number whose logarithm to be calculated and its type is System.Double.

base: It is the base of the logarithm of type System.Double.

Return Value: It returns the logarithm of val and its type is System.Double.

Note: The return value is always depend on the argument passed. Below table depicts the different cases:

val	base	Returned Value
val > 0	(0 < Base < 1) or(Base > 1)	logbase(val)
val < 0	any value	NaN
any value	base < 0	NaN
val != 1	base = 0	NaN
val != 1	base = +ve Infinity	NaN
val = NaN	any value	NaN
any value	base = NaN	NaN
any value	base = 1	NaN
val = 0	(0 < Base < 1)	+ve Infinity
val = 0	base > 1	-ve Infinity
val = +ve Infinity	(0 < Base < 1)	-ve Infinity
val = +ve Infinity	base > 1	+ve Infinity
val = 1	base = 0	0
val = 1	base = +ve Infinity	0

Example:

// C# program to demonstrate working
// of Math.Log(Double, Double) method
using System;

class Geeks {

    // Main Method
    public static void Main(String[] args)
    {

        // Here val = 1.3 and base is 0.3 then
        // output will be logarithm of given value
        Console.WriteLine(Math.Log(1.3, 0.3));

        // Here val is 0.5 and base > 1 then output
        // will be -0.5
        Console.WriteLine(Math.Log(0.5, 4));

        // Here val is 0.7 and base = 1 then output
        // will be NaN
        Console.WriteLine(Math.Log(0.7, 1));

        // Here val is 0.7 and base is NaN then output
        // will be NaN
        Console.WriteLine(Math.Log(0.7, Double.NaN));
    }
}

Output:

-0.217915440884381
-0.5
NaN
NaN

References:

• https://msdn.microsoft.com/en-us/library/x80ywz41(v=vs.110).aspx
• https://msdn.microsoft.com/en-us/library/hd50b6h5(v=vs.110).aspx