Bug #5715

+/-1 ** Bignum returns different results than +/-1 ** Fixnum

Added by John Firebaugh over 2 years ago. Updated about 1 year ago.

[ruby-core:41498]
Status:Closed
Priority:Normal
Assignee:Marc-Andre Lafortune
Category:core
Target version:2.0.0
ruby -v:ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin10.8.0] Backport:

Description

=begin
Consider (({1 ** expt})) and (({(-1) ** expt})).

When ((|expt|)) is a Fixnum, the result is always 1, -1, Rational(1/1), or Rational(-1,1) depending on the signs of the operands and parity of the exponent.

When ((|expt|)) is a Bignum, Float 1.0 is always returned. Either the behavior for Fixnum exponents should be followed exactly (preferred), or at the very least -1.0 should be returned when the base is -1 and the exponent is odd.

$VERBOSE = nil

wordsize = 8 * 1.size
fixnummax = 2 ** (wordsize - 2) - 1
fixnum
min = -2 ** (wordsize - 2)

[1, -1].each do |a|
[ 1, 2, fixnummax, fixnummax + 1, fixnummax + 2,
-1, -2, fixnum
min, fixnummin - 1, fixnummin - 2].each do |b|
puts "%5s ** %20s (%s) == %5s" % [a, b, b.class, a ** b]
end
end

Output:
1 ** 1 (Fixnum) == 1
1 ** 2 (Fixnum) == 1
1 ** 4611686018427387903 (Fixnum) == 1
1 ** 4611686018427387904 (Bignum) == 1
1 ** 4611686018427387905 (Bignum) == 1
1 ** -1 (Fixnum) == 1/1
1 ** -2 (Fixnum) == 1/1
1 ** -4611686018427387904 (Fixnum) == 1/1
1 ** -4611686018427387905 (Bignum) == 1.0 # Bug
1 ** -4611686018427387906 (Bignum) == 1.0 # Bug
-1 ** 1 (Fixnum) == -1
-1 ** 2 (Fixnum) == 1
-1 ** 4611686018427387903 (Fixnum) == -1
-1 ** 4611686018427387904 (Bignum) == 1
-1 ** 4611686018427387905 (Bignum) == -1
-1 ** -1 (Fixnum) == -1/1
-1 ** -2 (Fixnum) == 1/1
-1 ** -4611686018427387904 (Fixnum) == 1/1
-1 ** -4611686018427387905 (Bignum) == 1.0 # Bug
-1 ** -4611686018427387906 (Bignum) == 1.0 # Bug
=end


Related issues

Related to ruby-trunk - Bug #5713: Fixnum#** returns Infinity for 0 ** negative Bignum Closed 12/06/2011

Associated revisions

Revision 39063
Added by Marc-Andre Lafortune about 1 year ago

  • rational.c (nurat_expt): Deal with special cases for rationals 0, ±1 [bug #5713] [bug #5715]

Revision 39064
Added by Marc-Andre Lafortune about 1 year ago

  • numeric.c (fix_pow): Handle special cases when base is 0, -1 or +1 [Bug #5713] [Bug #5715]

History

#1 Updated by John Firebaugh over 2 years ago

The failing cases are the ones that go through the Rational(+/-1) ** Bignum code path, so this is closely related to #5713.

#2 Updated by Marc-Andre Lafortune over 2 years ago

  • Category set to core
  • Assignee set to Marc-Andre Lafortune

Just saw this update. Yes, as I stated, same issue as 5713. Yes the case for (-1) and bignum exponents can be fixed too. The case for 1 and 0 must be addressed for rational and float exponents too (but -1 will go to float for those)

#3 Updated by Shyouhei Urabe about 2 years ago

  • Status changed from Open to Assigned

#4 Updated by Marc-Andre Lafortune about 1 year ago

  • Target version set to 2.0.0

#5 Updated by Marc-Andre Lafortune about 1 year ago

  • Status changed from Assigned to Closed
  • % Done changed from 0 to 100

This issue was solved with changeset r39063.
John, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.


  • rational.c (nurat_expt): Deal with special cases for rationals 0, ±1 [bug #5713] [bug #5715]

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