Bug #17318: Raising float to the power of other issue

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 2.6.3
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[deXterbed (Manoj Mishra)]

Raising a negative float to another float results in a complex number. Interestingly, doing the same thing without using variables works fine!

Sample Snippet:

$ irb
2.6.3 :001 > x=-0.4790529833050308
=> -0.4790529833050308
2.6.3 :002 > y=0.9918032786885246
=> 0.9918032786885246
2.6.3 :003 > xy
=> (-0.48179173935576963+0.012409246172848264i)
2.6.3 :004 > -0.47905298330503080.9918032786885246
=> -0.4819515219418196

[deXterbed (Manoj Mishra)]

Formatting replaced the double * with a single * i put in the snippet above, please ignore

[mame (Yusuke Endoh)]

• Status changed from Open to Rejected

It is due to precedence of operators unary minus and **.

x**y calculates (-0.4790529833050308)**0.9918032786885246, and -0.4790529833050308**0.9918032786885246 calculates -(0.4790529833050308**0.9918032786885246).

irb(main):001:0> -(0.4790529833050308**0.9918032786885246)
=> -0.4819515219418196
irb(main):002:0> (-0.4790529833050308)**0.9918032786885246
=> (-0.48179173935576963+0.012409246172848266i)

[deXterbed (Manoj Mishra)]

mame (Yusuke Endoh) wrote in #note-2:

It is due to precedence of operators unary minus and **.

x**y calculates (-0.4790529833050308)**0.9918032786885246, and -0.4790529833050308**0.9918032786885246 calculates -(0.4790529833050308**0.9918032786885246).

irb(main):001:0> -(0.4790529833050308**0.9918032786885246)
=> -0.4819515219418196
irb(main):002:0> (-0.4790529833050308)**0.9918032786885246
=> (-0.48179173935576963+0.012409246172848266i)

Thanks for the explanation Yusuke Endoh,

that still raises the question, how exactly am i supposed to calculate x**y without ending up with a complex number? x and y are dynamic values, i can't use parentheses to force precedence. I could use "abs" method but shouldn't this be the default behaviour?

def raise(a, b)
  return -((a.abs)**b) if a < 0
  a**b
end

[Dan0042 (Daniel DeLorme)]

Also see #16677 w/r edge cases.

-0.4**0.9        #=> -0.4383832905540869
-0.4.to_f**0.9   #=> (-0.416927285116376+0.13546788683122327i)
-(0.4).to_f**0.9 #=> -0.4383832905540869

[mame (Yusuke Endoh)]

deXterbed (Manoj Mishra) wrote in #note-3:

def raise(a, b)
  return -((a.abs)**b) if a < 0
  a**b
end

I don't think that this should be the default behavior because raise(-1, 2) returns -1 but I expect the square of -1 to be 1.

[phluid61 (Matthew Kerwin)]

deXterbed (Manoj Mishra) wrote in #note-3:

that still raises the question, how exactly am i supposed to calculate x**y without ending up with a complex number?

Don't raise a negative number to a non-Integer power. That's just maths.

[gotoken (Kentaro Goto)]

That depends on your purpose or background of the problem. In non-math situations, power(a, b) should not return any ordinary number for given a<0 where power(a, b) stands for "a to the power of b" over real, just identical to your notation raise(a, b). In such cases, so the function may raise an exception or returns float::NAN if a<0 or a==0 && b==0 held.