On Feb 2, 2009, at 2:56 PM, Robert Rohde wrote:
On Mon, Feb 2, 2009 at 2:32 PM, Gregory Maxwell gmaxwell@gmail.com wrote:
On Sat, Jan 31, 2009 at 8:33 PM, Robert Rohde rarohde@gmail.com wrote:
This discussion is getting side tracked.
The real complaint here is that
{{#expr:(0.00007 * 1000 * 1000) mod 1000}} is giving 69 when it should give 70.
This is NOT a formatting issue, but rather it is bug in the #expr parser function, presumably caused by some kind of round-off error.
It's a bug in the user's understanding of floating point on computers, combined with % being (quite naturally) an operator on integers.
0.000070000… does not exist in your finite precision base-2 based computer.
I don't think it's reasonable for Mediawiki to include a full radix-n multi-precision floating point library in order to capture the expected your behavior for these cases, any more than it would be reasonable to expect it to contain a full computer algebra system so it could handle manipulations of irrationals precisely.
I've already written code that converts 4.9999999998 to 5 immediately before performing operations that explicitly expect integers by applying an explicit 1 part in 10^10 tolerance. That covers a wide range of cases that might be affected by round-off errors while adding little overhead.
-Robert Rohde
That sounds good to me, Robert. Could there be any downside to this? Can the concept be tried on the contents of my sandbox without messing with Wikipedia-proper? -Greg
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