Re: To get the accurate value of 1 - 0.999999999999999 ,how to implement the python algorithm ？

[Dave Angel]

On 10/08/2012 11:00 AM, Chris Angelico wrote:
> On Tue, Oct 9, 2012 at 1:48 AM, Dave Angel <d at davea.name> wrote:
>> import decimal
>> a = decimal.Decimal(4.3)
>> print(a)
>>
>> 5.0999999999999996447286321199499070644378662109375
> Ah, the delights of copy-paste :)
>
>> The Decimal class has the disadvantage that it's tons slower on any modern machine I know of...
> Isn't it true, though, that Python 3.3 has a completely new
> implementation of decimal that largely removes this disadvantage?
>
> ChrisA

I wouldn't know, I'm on 3.2.  However, I sincerely doubt if it's within
a factor of 100 of the speed of the binary float, at least on
pentium-class machines that do binary float in microcode. A dozen years
or so ago, when the IEEE floating point standard was still being formed,
I tried to argue the committee into including decimal in the standard
(which they did much later).  Had it been in the standard then, we MIGHT
have had decimal fp on chip as well as binary.  Then again, the standard
was roughly based on the already-existing Intel 8087, so maybe it was
just hopeless.

I guess it's possible that for some operations, the cost of the
byte-code interpreter and function lookup, etc. might reduce the
apparent penalty.  Has anybody done any timings?


-- 

DaveA