[yunqa.de] Re: Rounding of REAL and DISQLite3Api.DateTimeToJulianDate
- From: Delphi Inspiration <delphi@xxxxxxxx>
- To: yunqa@xxxxxxxxxxxxx
- Date: Wed, 25 Feb 2009 16:44:08 +0100
Edwin Yip wrote:
>But when the program gets the TDateTime values the precision is not only 4
>digits, but I can only use ROUND(X, 4) to compare the values after saving into
>the database in the way described in my original email...
The DateTimeToJulianDate function is implemented as a simple addition of two
floats. I tend to believe that this should contribute very little only to
precision loss. My testing shows that 12 digits of precision are well
preserved. The small loss is certainly not due to the code but to the CPU's
floating point unit. Not much can be done about this.
Various floating math libraries include dedicated functions to check for float
equality. Here is one which allows to pass the precision as a 3rd parameter,
i.e. 1e-6):
function FloatsEqual(const x, y, Delta: Extended): Boolean;
begin
Result := x = y;
if not Result then
try
if y = 0 then
Result := Abs(1 - y / x) <= Delta
else
Result := Abs(1 - x / y) <= Delta;
except
Result := False; // catch real rare overflow e.g. 1.0e3000/1.0e-3000
end
end;
If you need an 100% exact date and time representation with a guaranteed
accuracy down to the very second, you must use a separate date format like ISO
date (http://en.wikipedia.org/wiki/ISO_8601) or a Unix time
(http://en.wikipedia.org/wiki/Unix_time). Both formats are supported by the
DISQLite3 date and time functions.
Ralf
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