[Tim]
>> Try to spell out what you mean - precisely! - by "this". I can't do
>> that for you. For any plausible way of fleshing it out I've thought
>> of, the answer is "no".
[Marco Sulla <marco.sulla.pyt...@gmail.com>]
> Well, please, don't be so harsh. I'm trying to discuss to someone that
> co-created Python itself, it's not simple to me :-P
Sorry, I didn't mean to come off as harsh. I did mean to come off as
saying that details are important here - indeed, details are darned
near everything here. I'm not going to discuss vague wishlists in
this area.
>> The closest you can get to BigDecimal's behavior "by magic" in Python
>> is to set the context precision to its maximum allowed value.
> I think there's another "trick" to get the BigDecimal behaviour.
> If you read the Javadoc, it says that each operation has a default
> precision. For example, multiplication a*b has precision = a_scale +
> b_scale. So, in reality, also BigDecimal has a context with finite
> precision. The difference is that the default context has a variable
> precision, depending on the operation.
Scale is the conceptual exponent in BigDecimal - it has nothing to do
with digits of precision. For example, every normalized odd
BigDecimal integer has scale 0, regardless of whether the integer is
1, or 17 to the millionth power. A BigDecimal with significand `sig`
(an integer) and scale `s` (also an integer) represents the real
number sig * 10**-s.
That's where the motivation for the rule you paraphrased came from:
mathematically,
(a_sig * 10**-a_s) * (b_sig * 10**-b_s) = (a_sig * b_sig) * 10**-(a_s + b_s)
So "the natural" scale of the product is the sum of their scales (a_s
+ b_s), but the significand of the product is the infinite-precision
product of their signficands, no matter how many bits that takes.
For example, try this:
BigDecimal a = new BigDecimal("500").multiply(new
BigDecimal("100000000"));
System.out.println(a);
System.out.println(a.scale());
System.out.println(a.precision());
The output:
50000000000
0
11
The precision is 11 (decimal) digits, but the scale is 0. Change it to:
new BigDecimal("5e2").multiply(new BigDecimal("1e8"))
and the output changes to
5E+10
-10
1
> Could Python decimal have something similar, maybe by setting prec = -1?
No ;-)
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