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Math.Round Method (Double)

Updated: May 2010

Rounds a double-precision floating-point value to the nearest integral value.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

public static double Round(
	double a
)

Parameters

a
Type: System.Double

A double-precision floating-point number to be rounded.

Return Value

Type: System.Double
The integer nearest a. If the fractional component of a is halfway between two integers, one of which is even and the other odd, then the even number is returned. Note that this method returns a Double instead of an integral type.

The behavior of this method follows IEEE Standard 754, section 4. This kind of rounding is sometimes called rounding to nearest, or banker's rounding. It minimizes rounding errors that result from consistently rounding a midpoint value in a single direction.

To control the type of rounding used by the Round method, call the Math.Round(Double, MidpointRounding) overload.

Notes to Callers:

Because of the loss of precision that can result from representing decimal values as floating-point numbers or performing arithmetic operations on floating-point values, in some cases the Round(Double) method may not appear to round midpoint values to the nearest even integer. In the following example, because the floating-point value .1 has no finite binary representation, the first call to the Round(Double) method with a value of 11.5 returns 11 instead of 12.

using System;

public class Example
{
   public static void Main()
   {
      double value = 11.1;
      for (int ctr = 0; ctr <= 5; ctr++)    
         value = RoundValueAndAdd(value);

      Console.WriteLine();

      value = 11.5;
      RoundValueAndAdd(value);
   }

   private static double RoundValueAndAdd(double value)
   {
      Console.WriteLine("{0} --> {1}", value, Math.Round(value));
      return value + .1;
   }
}
// The example displays the following output: 
//       11.1 --> 11 
//       11.2 --> 11 
//       11.3 --> 11 
//       11.4 --> 11 
//       11.5 --> 11 
//       11.6 --> 12 
//        
//       11.5 --> 12

The following example demonstrates rounding to the nearest integer value.

using System;

class Program
{
    static void Main()
    {
    Console.WriteLine("Classic Math.Round in CSharp");
    Console.WriteLine(Math.Round(4.4)); // 4
    Console.WriteLine(Math.Round(4.5)); // 4
    Console.WriteLine(Math.Round(4.6)); // 5
    Console.WriteLine(Math.Round(5.5)); // 6
    }
}

The following code sample uses Round to assist in the computation of the inner angles of a given trapezoid.

	/// <summary> 
	/// The following class represents simple functionality of the trapezoid. 
	/// </summary> 
	class MathTrapezoidSample
	{
		private double m_longBase;
		private double m_shortBase;
		private double m_leftLeg;
		private double m_rightLeg;

		public MathTrapezoidSample(double longbase, double shortbase, double leftLeg, double rightLeg)
		{
			m_longBase = Math.Abs(longbase);
			m_shortBase = Math.Abs(shortbase);
			m_leftLeg = Math.Abs(leftLeg);
			m_rightLeg = Math.Abs(rightLeg);
		}

		private double GetRightSmallBase()
		{
			return (Math.Pow(m_rightLeg,2.0) - Math.Pow(m_leftLeg,2.0) + Math.Pow(m_longBase,2.0) + Math.Pow(m_shortBase,2.0) - 2* m_shortBase * m_longBase)/ (2*(m_longBase - m_shortBase));
		}

		public double GetHeight()
		{
			double x = GetRightSmallBase();
			return Math.Sqrt(Math.Pow(m_rightLeg,2.0) - Math.Pow(x,2.0));
		}

		public double GetSquare()
		{
			return GetHeight() * m_longBase / 2.0;
		}

		public double GetLeftBaseRadianAngle()
		{
			double sinX = GetHeight()/m_leftLeg;
			return Math.Round(Math.Asin(sinX),2);
		}

		public double GetRightBaseRadianAngle()
		{
			double x = GetRightSmallBase();
			double cosX = (Math.Pow(m_rightLeg,2.0) + Math.Pow(x,2.0) - Math.Pow(GetHeight(),2.0))/(2*x*m_rightLeg);
			return Math.Round(Math.Acos(cosX),2);
		}

		public double GetLeftBaseDegreeAngle()
		{
			double x = GetLeftBaseRadianAngle() * 180/ Math.PI;
			return Math.Round(x,2);
		}

		public double GetRightBaseDegreeAngle()
		{
			double x = GetRightBaseRadianAngle() * 180/ Math.PI;
			return Math.Round(x,2);
		}

		static void Main(string[] args)
		{
			MathTrapezoidSample trpz = new MathTrapezoidSample(20.0, 10.0, 8.0, 6.0);
			Console.WriteLine("The trapezoid's bases are 20.0 and 10.0, the trapezoid's legs are 8.0 and 6.0");
			double h = trpz.GetHeight();
			Console.WriteLine("Trapezoid height is: " + h.ToString());
			double dxR = trpz.GetLeftBaseRadianAngle();
			Console.WriteLine("Trapezoid left base angle is: " + dxR.ToString() + " Radians");
			double dyR = trpz.GetRightBaseRadianAngle();
			Console.WriteLine("Trapezoid right base angle is: " + dyR.ToString() + " Radians");
			double dxD = trpz.GetLeftBaseDegreeAngle();
			Console.WriteLine("Trapezoid left base angle is: " + dxD.ToString() + " Degrees");
			double dyD = trpz.GetRightBaseDegreeAngle();
			Console.WriteLine("Trapezoid left base angle is: " + dyD.ToString() + " Degrees");
		}
	}

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

May 2010

Added the Notes for Callers section.

Customer feedback.

February 2009

Expanded Return Value section.

Customer feedback.

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