On 2020-10-10, Peter J. Holzer <hjp-pyt...@hjp.at> wrote:
> On 2020-10-07 07:53:55 +0200, Marco Sulla wrote:
>> If you want to avoid float problems, you can use Decimal:
>
> Decimal doesn't avoid floating point problems, because it is a floating
> point format. For example:
> [...]
> >>> from decimal import *
> >>> a = Decimal(3)
> >>> a
> Decimal('3')
> >>> b = Decimal(1E50)
> >>> b
> Decimal('100000000000000007629769841091887003294964970946560')
> [...]
There are two problems with your code:
1. You meant Decimal('1e50'). What you typed creates a Decimal value
from the IEEE 64-bit floating point value closest to 1e50.
2. You need to increase the context precision. It defaults to 28,
and you're example needs it to be at least 51:
>>> getcontext().prec = 100
>>> a = Decimal(3)
>>> b = Decimal('1e50')
>>> c = Decimal(2)
>>> a + b - c - b
Decimal('1')
>>> b - b + a - c
Decimal('1')
>>> a + (b - b) - c
Decimal('1')
>>> a + b - b - c
Decimal('1')
Like other floating point systems, you still need to know what you're
doing if you want to get the "right" results.
--
Grant
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