From: Yusuke Endoh <redmine@...>
Date: 2010-07-13T23:59:45+09:00
Subject: [ruby-core:31256] [Bug #3563][Assigned] problems with exponentiation, cbrt

Issue #3563 has been updated by Yusuke Endoh.

Status changed from Open to Assigned
Assigned to set to Yusuke Endoh

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>
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http://redmine.ruby-lang.org/issues/show/3563

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