math.logb Method (Double)

mathlogb Method (Double)

Gets the unbiased exponent of the specified floating-point number.

Namespace: C
Assembly: C.math (in C.math.dll) Version: 1.0.0.0 (1.0.0.0)

Syntax

public static double logb(
	double number
)

Public Shared Function logb ( 
	number As Double
) As Double

public:
static double logb(
	double number
)

static member logb : 
        number : float -> float

Parameters

number: Type: SystemDouble
A floating-point number.

Return Value

Type: Double
The unbiased exponent of the specified floating-point number, or a special value if number is not normal or subnormal.

Remarks

The unbiased exponent is the integral part of the logarithm base 2 of number. The unbiased exponent is such that

number = significand(number) * 2^logb(number).

The return unbiased exponent is valid for all normal and subnormal numbers. Special values are treated as follows.

If number is ±0, NegativeInfinity is returned.
If number is infinite, PositiveInfinity is returned.
If number is NaN, NaN is returned.

If number is not zero, infinite, or NaN, the value returned is exactly equivalent to ilogb(number).

The value of the exponent returned by logb(Double) is always 1 less than the exponent retuned by frexp(Double, Int32) because of the different normalization requirements: for logb(Double), the normalized significand is in the interval [1, 2), but for frexp(Double, Int32), the normalized significand is in the interval [0.5, 1).

See logb in the C standard documentation.

Examples

C++

Copy

Assert.IsTrue(math.logb(1D) == 0D);
Assert.IsTrue(math.logb(System.Math.E) == 1D);
Assert.IsTrue(math.logb(1024D) == 10D);
Assert.IsTrue(math.logb(-2000D) == 10D);

Assert.IsTrue(math.logb(2D) == 1D);
Assert.IsTrue(math.logb(Math.Pow(2D, 56D)) == 56D);
Assert.IsTrue(math.logb(1.1D * Math.Pow(2D, -1074D)) == -1074D);
Assert.IsTrue(math.logb(Math.Pow(2D, -1075D)) == System.Double.NegativeInfinity);
Assert.IsTrue(math.logb(Math.Pow(2D, 1024D)) == System.Double.PositiveInfinity);
Assert.IsTrue(math.logb(Math.Pow(2D, 1023D)) == 1023D);
Assert.IsTrue(math.logb(2D * Math.Pow(2D, 102D)) == 103D);

Assert.IsTrue(math.logb(math.DBL_DENORM_MIN) == math.DBL_EXP_MIN - math.DBL_MANT_BITS);
Assert.IsTrue(math.logb(math.DBL_DENORM_MAX) == math.DBL_EXP_MIN - 1);
Assert.IsTrue(math.logb(math.DBL_MIN) == math.DBL_EXP_MIN);
Assert.IsTrue(math.logb(math.DBL_MAX) == math.DBL_EXP_MAX);

Assert.IsTrue(math.logb(System.Double.PositiveInfinity) == System.Double.PositiveInfinity);
Assert.IsTrue(math.logb(System.Double.NegativeInfinity) == System.Double.PositiveInfinity);
Assert.IsTrue(math.logb(0D) == System.Double.NegativeInfinity);
Assert.IsTrue(math.logb(-0D) == System.Double.NegativeInfinity);
Assert.IsTrue(System.Double.IsNaN(math.logb(System.Double.NaN)));
Assert.IsTrue(System.Double.IsNaN(math.logb(-System.Double.NaN)));

Assert.IsTrue(math.logb(1D) = 0D);
Assert.IsTrue(math.logb(System.Math.E) = 1D);
Assert.IsTrue(math.logb(1024D) = 10D);
Assert.IsTrue(math.logb(-2000D) = 10D);

Assert.IsTrue(math.logb(2D) = 1D);
Assert.IsTrue(math.logb(Math.Pow(2D, 56D)) = 56D);
Assert.IsTrue(math.logb(1.1D * Math.Pow(2D, -1074D)) = -1074D);
Assert.IsTrue(math.logb(Math.Pow(2D, -1075D)) = System.Double.NegativeInfinity);
Assert.IsTrue(math.logb(Math.Pow(2D, 1024D)) = System.Double.PositiveInfinity);
Assert.IsTrue(math.logb(Math.Pow(2D, 1023D)) = 1023D);
Assert.IsTrue(math.logb(2D * Math.Pow(2D, 102D)) = 103D);

Assert.IsTrue(math.logb(math.DBL_DENORM_MIN) = math.DBL_EXP_MIN - math.DBL_MANT_BITS);
Assert.IsTrue(math.logb(math.DBL_DENORM_MAX) = math.DBL_EXP_MIN - 1);
Assert.IsTrue(math.logb(math.DBL_MIN) = math.DBL_EXP_MIN);
Assert.IsTrue(math.logb(math.DBL_MAX) = math.DBL_EXP_MAX);

Assert.IsTrue(math.logb(System.Double.PositiveInfinity) = System.Double.PositiveInfinity);
Assert.IsTrue(math.logb(System.Double.NegativeInfinity) = System.Double.PositiveInfinity);
Assert.IsTrue(math.logb(0D) = System.Double.NegativeInfinity);
Assert.IsTrue(math.logb(-0D) = System.Double.NegativeInfinity);
Assert.IsTrue(System.Double.IsNaN(math.logb(System.Double.NaN)));
Assert.IsTrue(System.Double.IsNaN(math.logb(-System.Double.NaN)));

Reference

math Class

logb Overload

C Namespace