Rounding Issue using Math.Round

Question

Module Module1
Public Sub Main()
    Dim values() As Double = {43.523, 12.65, 43.565}
    For Each value As Double In values
        Console.WriteLine("{0} --> {1}", value, Math.Round(value, 2))
    Next
    Console.ReadLine()
End Sub
End Module

The above code results as

43.523 --> 43.52
12.65 --> 12.65
43.565 --> 43.56

I need 43.565 --> 43.57 and not 43.565 --> 43.56. But i still need the other 43.523 --> 43.52 and 12.65 --> 12.65 rounded as is.

But note that "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." For `Round(43.565,2,mode)`, it does, since although the `Double` closest to 43.565 is smaller than that, multiplying by 100 yields exactly 4356.5, so the value rounded to an integer _is_ a midpoint. But for 2.135, you get 2.13, since the rounded value isn't a midpoint. — Daniel Fischer, Aug 30 '12 at 21:54

score 8 · Accepted Answer · edited Aug 30 '12 at 21:48

Firstly, if exact decimal values are of concern to you, you should consider using Decimal instead of Double. In particular, 43.565 isn't exactly representable as a Double to start with.

However, if you want to specify the behaviour for "midpoints" (i.e. where it could reasonably round up or down), use the overload with a MidpointRounding parameter:

Console.WriteLine("{0} --> {1}", value, _
                  Math.Round(value, 2, MidpointRounding.AwayFromZero))

score 4 · Answer 2 · answered Aug 30 '12 at 19:49

4

You can use:

Math.Round(value, 2, MidpointRounding.AwayFromZero)

For details, see the overload of Math.Round which accepts a MidpointRounding.

answered Aug 30 '12 at 19:49

Reed Copsey

554,122
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1,158
1,373

score 1 · Answer 3 · answered Aug 30 '12 at 19:50

1

Use

Math.Round(value, 2, System.MidpointRounding.AwayFromZero)

answered Aug 30 '12 at 19:50

Olivier Jacot-Descombes

104,806
13
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188

score 1 · Answer 4 · answered Aug 30 '12 at 19:51

1

Check out the MidpointRounding parameter:

Math.Round(43.565, 2, MidpointRounding.AwayFromZero)

Should give you 43.57

answered Aug 30 '12 at 19:51

Mike Christensen

88,082
50
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Rounding Issue using Math.Round

4 Answers4