Bug #3563
closed
problems with exponentiation, cbrt
Description
=begin
nothing required¶
(-27)**(Rational(1,3)) #=> (1.5000000000000004+2.598076211353316i)
I would expect that this would return -3
Complex(1.5000000000000004,2.598076211353316)**3 #=> (-27.000000000000007+1.2434497875801753e-14i)
I can understand how this might happen given round off errors, but the following identity should be true
Math.cbrt(-27) == (-27)**(Rational(1,3)) #=> false
Also note that if you do this...
require 'complex'
Math.cbrt(-27) #=> (1.5000000000000004+2.598076211353316i)
=end
Updated by mame (Yusuke Endoh) over 14 years ago
- Status changed from Open to Assigned
- Assignee set to mame (Yusuke Endoh)
=begin
Hi,
2010/7/13 Kevin Olbrich redmine@ruby-lang.org:
(-27)**(Rational(1,3)) #=> (1.5000000000000004+2.598076211353316i)
I would expect that this would return -3
I think this is not a bug. 1.9.1 and 1.9.2 should be changed.
But I understand your expectation. To change this behavior in the
future, I propose to add a note to rdoc of Rational#**:
diff --git a/rational.c b/rational.c
index f5a6d26..734131b 100644
--- a/rational.c
+++ b/rational.c
@@ -907,6 +907,9 @@ nurat_fdiv(VALUE self, VALUE other)
*
- Performs exponentiation.
-
- When the solution is underspecified, it is not specified which
-
- solution this method returns.
-
- For example:
- Rational(2) ** Rational(3) #=> (8/1)
Also note that if you do this...
require 'complex'
Math.cbrt(-27) #=> (1.5000000000000004+2.598076211353316i)
complex.rb is deprecated, but mathn.rb causes the same.
I think this is a bug of cmath:
diff --git a/lib/cmath.rb b/lib/cmath.rb
index b23dac2..976f269 100644
--- a/lib/cmath.rb
+++ b/lib/cmath.rb
@@ -85,7 +85,7 @@ module CMath
end
def cbrt(z)
- if z.real? and z >= 0
- if z.real?
cbrt!(z)
else
Complex(z) ** (1.0/3)
Unless there is objection, I'll commit these patches to both trunk
and ruby_1_9_2.
--
Yusuke Endoh mame@tsg.ne.jp
=end
Updated by kolbrich (Kevin Olbrich) over 14 years ago
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I'm fine with adding a comment, however the one you added does not really clarify the matter (at least not to me).
=end
Updated by mame (Yusuke Endoh) over 14 years ago
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Umm. Could you make a draft?
2010/7/14 Kevin Olbrich redmine@ruby-lang.org:
I'm fine with adding a comment, however the one you added does not really clarify the matter (at least not to me).
--
Yusuke Endoh mame@tsg.ne.jp
=end
Updated by mame (Yusuke Endoh) over 14 years ago
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I'll commit only the patch for cmath.rb and close this ticket.
If you (or anyone) show me a draft in a few days, I'll import it.
But if you don't, we will release 1.9.2 without any note, which
may make it difficult to change the behavior in 1.9.3 or later.
2010/7/14 Yusuke ENDOH mame@tsg.ne.jp:
Umm. Could you make a draft?
2010/7/14 Kevin Olbrich redmine@ruby-lang.org:
I'm fine with adding a comment, however the one you added does not really clarify the matter (at least not to me).
--
Yusuke Endoh mame@tsg.ne.jp
--
Yusuke Endoh mame@tsg.ne.jp
=end
Updated by mame (Yusuke Endoh) over 14 years ago
- Status changed from Assigned to Closed
- % Done changed from 0 to 100
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This issue was solved with changeset r28698.
Kevin, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
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