Need help rounding to 2 decimal places

Question

Why is it that when I do the following     Math Round 0 75  1  MidpointRounding AwayFromZero    I get 0 8  but when I do the following     Math Round 0 575  2  MidpointRounding AwayFromZero    I don t get 0 58  Instead I get 0 57  I want anything that is 5 and up rounding up  so 0 575 should be 0 58

User · Answer

The System Math Round method uses the Double structure  which  as others have pointed out  is prone to floating point precision errors  The simple solution I found to this problem when I encountered it was to use the System Decimal Round method  which doesn t suffer from the same problem and doesn t require redifining your variables as decimals   Decimal Round 0 575  2  MidpointRounding AwayFromZero    Result  0 58

User · Answer

It is caused by a lack of precision with doubles   decimals  i e  - the function will not always give the result you expect    See the following link  MSDN on Math Round  Here is the relevant quote      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

User · Answer

The problem will be that you cannot represent 0 575 exactly as a binary floating point number  eg a double   Though I don t know exactly it seems that the representation closest is probably just a bit lower and so when rounding it uses the true representation and rounds down   If you want to avoid this problem then use a more appropriate data type  decimal will do what you want   Math Round 0 575M  2  MidpointRounding AwayFromZero    Result  0 58  The reason that 0 75 does the right thing is that it is easy to represent in binary floating point since it is simple 1 2   1 4  ie 2 -1  2 -2   In general any finite sum of powers of two can be represented in binary floating point  Exceptions are when your powers of 2 span too great a range  eg 2 100 2 is not exactly representable    Edit to add    Formatting doubles for output in C  might be of interest in terms of understanding why its so hard to understand that 0 575 is not really 0 575  The DoubleConverter in the accepted answer will show that 0 575 as an Exact String is 0 5749999999999999555910790149937383830547332763671875 You can see from this why rounding give 0 57

[.net] Need help rounding to 2 decimal places

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