Round Method (Double, Int32, MidpointRounding)

Math.Round Method (Double, Int32, MidpointRounding)

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Rounds a double-precision floating-point value to the specified number of fractional digits. A parameter specifies how to round the value if it is midway between two other numbers.

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

'Declaration
Public Shared Function Round ( _
	value As Double, _
	digits As Integer, _
	mode As MidpointRounding _
) As Double

Parameters

value
Type: System.Double
A double-precision floating-point number to be rounded.
digits
Type: System.Int32
The number of fractional digits in the return value.
mode
Type: System.MidpointRounding
Specification for how to round value if it is midway between two other numbers.

Return Value

Type: System.Double
The number nearest to value that has a number of fractional digits equal to digits. If value has fewer fractional digits than digits, value is returned unchanged.

ExceptionCondition
ArgumentOutOfRangeException

digits is less than 0 or greater than 15.

ArgumentException

mode is not a valid value of System.MidpointRounding.

The digits parameter specifies the number of fractional digits in the return value and ranges from 0 to 15. If digits is zero, then an integer is returned.

The mode parameter controls how value is rounded if the value to the right of the digits position is halfway between x and x+1, where x represents the value in the digits position. mode can have one of the following two values:

  • MidpointRounding.ToEven. If the digit in the digits position is odd, it is changed to an even digit. Otherwise, it is left unchanged. This behavior follows IEEE Standard 754, section 4. It 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.

  • MidpointRounding.AwayFromZero. The digit in the digits position is always rounded up to the next digit. This is the most commonly known rounding method. It is known as symmetric arithmetic rounding.

The maximum total number of integral and fractional digits that can be returned is 15. If the rounded value contains more than 15 digits, the 15 most significant digits are returned. If the rounded value contains 15 or fewer digits, the integral digits and as many fractional digits as the digits parameter specifies are returned.

If the value of value is Double.NaN, the method returns Double.NaN. If the value of value is Double.PositiveInfinity or Double.NegativeInfinity, the method returns Double.PositiveInfinity or Double.NegativeInfinity, respectively.

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, Int32, MidpointRounding) method may not appear to round midpoint values as specified by the mode parameter. This is illustrated in the following example, where 2.135 is rounded to 2.13 instead of 2.14. This occurs because internally the method multiplies value by 10digits, and the multiplication operation in this case suffers from a loss of precision.


Module Example
   Public Sub Demo(outputBlock As System.Windows.Controls.TextBlock)
      Dim values() As Double = { 2.125, 2.135, 2.145, 3.125, 3.135, 3.145 }
      For Each value As Double In values
         outputBlock.Text += String.Format("{0} --> {1}", value, 
                           Math.Round(value, 2, MidpointRounding.AwayFromZero)) + Environment.NewLine
      Next
   End Sub
End Module
' The example displays the following output:
'       2.125 --> 2.13
'       2.135 --> 2.13
'       2.145 --> 2.15
'       3.125 --> 3.13
'       3.135 --> 3.14
'       3.145 --> 3.15


The following code example demonstrates how to use the Round method with the MidpointRounding enumeration. Although the code example rounds decimal numbers, the Round method rounds double-precision floating-point numbers in a similar way.

Windows Phone OS

Supported in: 8.1, 8.0

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