Forum: Ruby-core [ruby-trunk - Bug #8826][Open] BigDecimal#div and #quo different behavior and inconsistencies

E897c9afcfcec3fef9ed461438ad3d38?d=identicon&s=25 Földes László (karatedog)
on 2013-08-27 20:44
(Received via mailing list)
Issue #8826 has been reported by karatedog (Földes László).

----------------------------------------
Bug #8826: BigDecimal#div and #quo different behavior and
inconsistencies
https://bugs.ruby-lang.org/issues/8826

Author: karatedog (Földes László)
Status: Open
Priority: Normal
Assignee:
Category:
Target version:
ruby -v: ruby 2.0.0p247 (2013-06-27 revision 41674) [i686-linux]
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN


BigDecimal's #div and #quo method behave differently despite the #div
documentation says: "See BigDecimal#quo"

#div returns Fixnum if there is no precision argument for #div (this
parameter is not documented):

2.0.0-p247 :018 > BigDecimal(5).div(5).class
 => Fixnum
2.0.0-p247 :031 > BigDecimal(5).div(5.1).class
 => Fixnum

#div returns Fixnum even for a Float argument:
2.0.0-p247 :118 > BigDecimal(5).div(5.01)
 => 0

It returns Fixnum even if every argument is BigDecimal:
2.0.0-p247 :043 > BigDecimal(5).div(BigDecimal(5.1,5)).class
 => Fixnum

When provided the precision argument, #div returns BigDecimal:
2.0.0-p247 :036 > BigDecimal(5).div(5,8).class
 => BigDecimal
2.0.0-p247 :131 > BigDecimal(5).div(BigDecimal(5.1,5),8).class
 => BigDecimal

But first argument cannot be Float along with precision:
2.0.0-p247 :032 > BigDecimal(5).div(5.1,8).class
ArgumentError: Float can't be coerced into BigDecimal without a
precision
  from (irb):32:in `div'
  from (irb):32
  from /home/karatedog/.rvm/rubies/ruby-2.0.0-p247/bin/irb:13:in
`<main>'

Whereas #quo does not accept a precision argument and returns BigDecimal
(hence no configurable precision here, although the documentation says
that #quo applies round operation):
2.0.0-p247 :121 > BigDecimal(5).quo(5.01)
 => #<BigDecimal:8bacf68,'0.9980039920 1596806387 225549E0',27(45)>

Circumventing the precision with class method does not work on #quo,
it's like the limit is maxed:
2.0.0-p247 :135 > BigDecimal::limit(5)
 => 5
2.0.0-p247 :136 > BigDecimal(1).quo(3)
 => #<BigDecimal:899c778,'0.33333E0',9(36)>
2.0.0-p247 :080 > BigDecimal::limit(50)
 => 5
2.0.0-p247 :081 > BigDecimal(1).quo(3)
 => #<BigDecimal:8a92d94,'0.3333333333 33333333E0',18(36)>

Precision does not seem to be automatic:
2.0.0-p247 :141 > BigDecimal::limit(500)
 => 100
2.0.0-p247 :142 > BigDecimal(1).quo(229)
 => #<BigDecimal:8be67f4,'0.4366812227 074236E-2',18(36)>
229 is a full period prime, its reciprocal yields 228 fractional digits
before repetition.

Whereas #div's precision can be larger than #div's:
2.0.0-p247 :109 > BigDecimal(1).div(3,19)
 => #<BigDecimal:8acb2d4,'0.3333333333 333333333E0',27(54)>

And for 229:
2.0.0-p247 :144 > BigDecimal(1).div(229,250)
 => #<BigDecimal:8bc8b28,'0.4366812227 0742358078 6026200873 3624454148
4716157205 2401746724 8908296943 2314410480 3493449781 6593886462
8820960698 6899563318 7772925764 1921397379 9126637554 5851528384
2794759825 3275109170 3056768558 9519650655 0218340611 3537117903
9301310043 6681222707...

Expected behavior:
- One division method to rule them all :-) (#Division)
- Never truncate a result (aka no Fixnum/Bignum as result). If someone
uses BigDecimal, they probably wanted large precision instead of
truncating the results by default.
- #Division should accept Float, Rational, String, Complex, Integer,
BigDecimal as divisor, even Float w/o precision. (This is intended as a
full list of acceptable classes, #div and #quo can already take
different classes as arguments).
- #Division should accept a precision argument which would override
::Limit (as this happens in many instance method), this argument is
optional. Without precision argument, use ::Limit

as for now proper calculation only happens if:
- method is #div
- the divisor is converted to BigDecimal
- a precision argument is given to #div
054b5f6b8afdd5f6190bad08e46cd782?d=identicon&s=25 zzak (Zachary Scott) (Guest)
on 2013-09-03 06:23
(Received via mailing list)
Issue #8826 has been updated by zzak (Zachary Scott).

Category set to ext
Status changed from Open to Assigned
Assignee set to mrkn (Kenta Murata)

The documentation for BigDecimal#div is aliases under #quo, sorry for
the confusion. I will fix this!

@mrkn Can you also comment?
----------------------------------------
Bug #8826: BigDecimal#div and #quo different behavior and
inconsistencies
https://bugs.ruby-lang.org/issues/8826#change-41553

Author: karatedog (Földes László)
Status: Assigned
Priority: Normal
Assignee: mrkn (Kenta Murata)
Category: ext
Target version:
ruby -v: ruby 2.0.0p247 (2013-06-27 revision 41674) [i686-linux]
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN


BigDecimal's #div and #quo method behave differently despite the #div
documentation says: "See BigDecimal#quo"

#div returns Fixnum if there is no precision argument for #div (this
parameter is not documented):

2.0.0-p247 :018 > BigDecimal(5).div(5).class
 => Fixnum
2.0.0-p247 :031 > BigDecimal(5).div(5.1).class
 => Fixnum

#div returns Fixnum even for a Float argument:
2.0.0-p247 :118 > BigDecimal(5).div(5.01)
 => 0

It returns Fixnum even if every argument is BigDecimal:
2.0.0-p247 :043 > BigDecimal(5).div(BigDecimal(5.1,5)).class
 => Fixnum

When provided the precision argument, #div returns BigDecimal:
2.0.0-p247 :036 > BigDecimal(5).div(5,8).class
 => BigDecimal
2.0.0-p247 :131 > BigDecimal(5).div(BigDecimal(5.1,5),8).class
 => BigDecimal

But first argument cannot be Float along with precision:
2.0.0-p247 :032 > BigDecimal(5).div(5.1,8).class
ArgumentError: Float can't be coerced into BigDecimal without a
precision
  from (irb):32:in `div'
  from (irb):32
  from /home/karatedog/.rvm/rubies/ruby-2.0.0-p247/bin/irb:13:in
`<main>'

Whereas #quo does not accept a precision argument and returns BigDecimal
(hence no configurable precision here, although the documentation says
that #quo applies round operation):
2.0.0-p247 :121 > BigDecimal(5).quo(5.01)
 => #<BigDecimal:8bacf68,'0.9980039920 1596806387 225549E0',27(45)>

Circumventing the precision with class method does not work on #quo,
it's like the limit is maxed:
2.0.0-p247 :135 > BigDecimal::limit(5)
 => 5
2.0.0-p247 :136 > BigDecimal(1).quo(3)
 => #<BigDecimal:899c778,'0.33333E0',9(36)>
2.0.0-p247 :080 > BigDecimal::limit(50)
 => 5
2.0.0-p247 :081 > BigDecimal(1).quo(3)
 => #<BigDecimal:8a92d94,'0.3333333333 33333333E0',18(36)>

Precision does not seem to be automatic:
2.0.0-p247 :141 > BigDecimal::limit(500)
 => 100
2.0.0-p247 :142 > BigDecimal(1).quo(229)
 => #<BigDecimal:8be67f4,'0.4366812227 074236E-2',18(36)>
229 is a full period prime, its reciprocal yields 228 fractional digits
before repetition.

Whereas #div's precision can be larger than #div's:
2.0.0-p247 :109 > BigDecimal(1).div(3,19)
 => #<BigDecimal:8acb2d4,'0.3333333333 333333333E0',27(54)>

And for 229:
2.0.0-p247 :144 > BigDecimal(1).div(229,250)
 => #<BigDecimal:8bc8b28,'0.4366812227 0742358078 6026200873 3624454148
4716157205 2401746724 8908296943 2314410480 3493449781 6593886462
8820960698 6899563318 7772925764 1921397379 9126637554 5851528384
2794759825 3275109170 3056768558 9519650655 0218340611 3537117903
9301310043 6681222707...

Expected behavior:
- One division method to rule them all :-) (#Division)
- Never truncate a result (aka no Fixnum/Bignum as result). If someone
uses BigDecimal, they probably wanted large precision instead of
truncating the results by default.
- #Division should accept Float, Rational, String, Complex, Integer,
BigDecimal as divisor, even Float w/o precision. (This is intended as a
full list of acceptable classes, #div and #quo can already take
different classes as arguments).
- #Division should accept a precision argument which would override
::Limit (as this happens in many instance method), this argument is
optional. Without precision argument, use ::Limit

as for now proper calculation only happens if:
- method is #div
- the divisor is converted to BigDecimal
- a precision argument is given to #div
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