VB.Net Rounding error

Assuming you KNOW that you started with an integer . . .

double a = System.Math.Round(1580/11.0, 4);
double Delta = 0.00001;
int b = (int)(System.Math.Floor((a * 11.0) + Delta));

In other words, do your multiplication by 11 - it won't come out as an exact integer (floating point arithmetic rarely does) - add a small amount to make sure you are just over, not just under, an integer - and take Floor to get next lower integer.

[sorry, this is C# but the functions / techniques are the same in VB, just change the syntax]

If you didn't start with an integer, you need to know how many decimal places you had - and scale values accordingly - so if 3dp (e.g. 1580.123) do:

double a = System.Math.Round(1580.123/11.0, 4);
double Delta = 0.00001;
double b = System.Math.Floor((a * 1000 * 11.0) + Delta) / 1000;

If you've rounded your intermediate result to 4dp but know you started with a value that wasn't necessarily integral and may have had up to 8 valid dp, you CAN'T, even theoretically, retrieve the original value accurately to 8dp.

e.g. 1580.00000001 and 1580.00000002 come out the same if divided by 11 and then rounded to 4dp - so you cannot distinguish between them when reversing the operation, so one of them must come out 'wrong'.

You seem to be confused. The result of your 1^st operation is rounded to 4 decimal places and you really want to have the result of the 2^nd operation which depends on the first to have a precision of 8 decimal places. That doesn't really make any sens, but you could try to explain why you are doing this.
As to the result not being equal to 1580. The result of the first operation was rounded to four decimal places when it was really a periodic decimal fraction:[^] .63636363 etc.. So once you start rounding you'll most likely won't get the same result where you started from. This is not an error but inherent to the problems encountered when doing rounding.
What you can do is separate GUI state from you logic. You can always do rounding when displaying the results, but keep the original result for doing further calculations.

Regards,

-MRB

I still think you're confused.

You're confused when you say "I want it rounded". I know almost no cases when anyone needs anything rounded. Rounding introduces "rounding errors"; and this is a very delicate matter: rounding errors, even though each one is acceptable, can accumulate, and so on.

I think you need the rounded "output", that is, only string presentation of the value should be rounded. This is completely different thing.
All you need to know is formatting. Please see how string.Format and object.ToString methods work with formatting.
See:
http://msdn.microsoft.com/en-us/library/txafckwd.aspx[^],
http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx[^],
http://msdn.microsoft.com/en-us/library/0c899ak8.aspx[^].

—SA