Bug #16518
closed
Should we rationalize Rational's numerator automatically?
Added by st0012 (Stan Lo) over 4 years ago.
Updated almost 4 years ago.
Description
In this Rails issue we found that using BigDecimal
as a Rational
's numerator can have inconsistent behaviors when the denominator gets bigger. For example:
> 1 == BigDecimal(1)
=> true
> Rational(1, 1) == Rational(BigDecimal(1), 1)
=> true
> Rational(1, 6) == Rational(BigDecimal(1), 6)
=> true
> Rational(1, 60) == Rational(BigDecimal(1), 60)
=> false
So my question is, is this behavior expected?
If it's not expected, do we have a plan to fix it?
If it is, does it make sense to manually rationalize the numerator before passing it into the Rational
call, in order to get a consistent result?
- Assignee set to mrkn (Kenta Murata)
This seems due to the fact Rational(BigDecimal, Integer)
does not return a Rational but a BigDecimal!
That seems highly surprising to me, any Foo()
"constructor" should return a Foo
, isn't it?
Also a Float
as numerator correctly produces a Rational, so the bug seems to be with BigDecimal as a numerator:
> Rational(BigDecimal(1), 2).class
=> BigDecimal
> Rational(BigDecimal(1).to_f, 2).class
=> Rational
Thanks for the quick response!
I also found that it returns BigDecimal
if any of its arguments is a BigDecimal
:
irb(main):005:0> Rational(BigDecimal(1), 1).class
=> BigDecimal
irb(main):006:0> Rational(BigDecimal(1), BigDecimal(1)).class
=> BigDecimal
irb(main):007:0> Rational(0, BigDecimal(1)).class
=> BigDecimal
irb(main):008:0>
After a quick trace of the source code, the problem is BigDecimal is a subclass of Numeric but defined in Ruby.
The nurat_conver
method check for argument type and convert to Rational, but the Numeric doesn't.
Ref: https://github.com/ruby/ruby/blob/5275d8bf4c43db9f057d24a26cf33ecd69f8b345/rational.c#L2632
When it calls the f_div
method, it calculates it directly without converting them to Rational.
We can simply force convert the result, but I didn't think it is the best way:
return to_rational(f_div(a1, a2));
- Status changed from Open to Assigned
- Status changed from Assigned to Closed
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