Bug #8826: BigDecimal#div and #quo different behavior and inconsistencies

• Status: Third Party's Issue
• Assignee: mrkn (Kenta Murata)
• Target version: -
• ruby -v: ruby 2.0.0p247 (2013-06-27 revision 41674) [i686-linux]
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
• Tags: ext

[karatedog (Földes László)]

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

[zzak (zzak _)]

• 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 (Kenta Murata) Can you also comment?

[mrkn (Kenta Murata)]

• Description updated (diff)
• Backport changed from 1.9.3: UNKNOWN, 2.0.0: UNKNOWN to 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN

[mrkn (Kenta Murata)]

Sorry for the late response.
I will fix this bug in the version of bigdecimal after releasing 1.3.

[karatedog (Földes László)]

How should #div work? The above behavior is still in Ruby 2.5.1.

[jeremyevans0 (Jeremy Evans)]

• Related to Bug #13754: bigdecimal with lower precision that Float added

[mrkn (Kenta Murata)]

• Status changed from Assigned to Third Party's Issue

The behavior of BigDecimal#div to return an Integer isn't a bug but the specification of Numeric#div. Float#div also always returns an Integer.

It is a bug that BigDecimal#div doesn't accept a Float without precision. I'll fix it in https://github.com/ruby/bigdecimal/issues/212.

BigDecimal#quo should be different from BigDecimal#div as Numeric#quo is different from Numeric#div. As Float#quo returns a Float, it is natural that BigDecimal#quo returns a BigDecimal.

BigDecimal#quo must accept a precision of the result like BigDecimal#div. I'll fix it in https://github.com/ruby/bigdecimal/issues/214.