Alternative to Decimal type
Nick Craig-Wood
nick at craig-wood.com
Thu Jun 12 04:01:49 CEST 2008
Frank Millman <frank at chagford.com> wrote:
> Thanks to all for the various replies. They have all helped me to
> refine my ideas on the subject. These are my latest thoughts.
>
> Firstly, the Decimal type exists, it clearly works well, it is written
> by people much cleverer than me, so I would need a good reason not to
> use it. Speed could be a good reason, provided I am sure that any
> alternative is 100% accurate for my purposes.
>
> My approach is based on expressing a decimal number as a combination
> of an integer and a scale, where scale means the number of digits to
> the right of the decimal point.
>
> Therefore 0.04 is integer 4 with scale 2, 1.1 is integer 11 with scale
> 1, -123.456 is integer -123456 with scale 3. I am pretty sure that any
> decimal number can be accurately represented in this form.
>
> All arithmetic is carried out using integer arithmetic, so although
> there may be rounding differences, there will not be the spurious
> differences thrown up by trying to use floats for decimal
> arithmetic.
I used an identical scheme in a product some time ago (written in C
not python). It was useful because it modelled the problem domain
exactly.
You might want to investigate the rational class in gmpy which might
satisfy your requirement for exact arithmetic :-
>>> import gmpy
>>> gmpy.mpq(123,1000)
mpq(123,1000)
>>> a = gmpy.mpq(123,1000)
>>> b = gmpy.mpq(12,100)
>>> a+b
mpq(243,1000)
>>> a*b
mpq(369,25000)
>>> a/b
mpq(41,40)
>>>
It is also *very* fast being written in C.
--
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick
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