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- Subject: Re: Possible improvements to the LNUM patch?
- From: David Given <dg@...>
- Date: Tue, 15 Apr 2008 01:02:07 +0100
> You will not have accuracy problems until you have a great many
> transactions. I wrote a quick lua program to see what the worst case would
> be, and it will never happen with my program unless you have (I forget how
> many - an extremely large number) of transactions.
It depends very much what you do. The errors show up immediately when
rounding edge cases.
70 cents + 5% == 0.70 * 1.05 = 0.735, which should round up to 0.74, but:
function round(n) return string.format("%.2f", n) end
print(round(0.70 * 1.05)) -> "0.73"
print(round(1.30 * 1.05)) -> "1.37" (but should be 1.36)
There are other problems, too; consider 1.0 % 0.1. The result should be
0, right? Try it:
print(1.0 % 0.1) -> "0.1"
All these operations and edge cases actually have *defined* behaviour in
the financial world. I gather it's actually *illegal* to use floating
point for some applications...
Here's a good explanation, although it is in Fortran:
┌─── ｄｇ＠ｃｏｗｌａｒｋ．ｃｏｍ ───── http://www.cowlark.com ─────
│ "I have always wished for my computer to be as easy to use as my
│ telephone; my wish has come true because I can no longer figure out
│ how to use my telephone." --- Bjarne Stroustrup
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