Is there a bug in Ruby or what is it?

Question

How to calculate the cubic root of a negative number and get the correct answer? A more general question concerns the all odd powers of negative numbers.

https://code.sololearn.com/c4pGf2PQ73Wd/?ref=app

Answer 1

Gordie My system crashed while gathering this so it's a little late, but I had traced what Ruby's producing:

First, -8.coerce(1.0/3) indicates the common class is Float.

Then, Float's power function sees a negative value + precision-losing power, so converts to complex [link 1]
1252: rb_float_pow()
1266: if (dx < 0 && dy != round(dy))
return num_funcall1(rb_complex_raw1(x), idPow, y);

(trace... rb_complex_raw1 (intern.h)
-> rb_complex_raw(x, INT2FIX(0)) # complex.c
-> nucomp_s_new_internal(rb_cComplex, x, y)
267: simply sets the real and imag components to x,y
...trace)

Then the exponent function in complex.c [link 2]
851: nucomp_expt(self, other)
866: if other is complex...
# calculate nr and ntheta
878: return complex.polar(Complex, nr, ntheta)
[or, edit]
914: if other is real...
923: return complex.polar(these defines come from [3])

I'm including this because you might want to see how the function calculates the final result (it's short).

[1] https://github.com/ruby/ruby/blob/trunk/numeric.c
[2] https://github.com/ruby/ruby/blob/trunk/complex.c
[3] https://github.com/ruby/date/blob/master/ext/date/date_strptime.c

Answer 2

It was more for your benefit. It's easier for people to answer if it's a fresh question ☺ You can still link to the old question for reference.

Answer 3

Gordie
Thank you very much!

Answer 4

Gordie
Thank you very much!

Answer 5

You need to create another question, rather than post a question in a question.

Answer 6

Ok, no problems. I just thought that these topics are closely related.

Answer 7

Thank you!

Answer 8

Someone else had this problem too:
http://compsci.ca/v3/viewtopic.php?t=15472

Answer 9

Thank you for information!

Answer 10

Is it printing out imaginary numbers as a cube root? Where i^2 = -1

Answer 11

Check excepted answer of cube root of -8
https://math.stackexchange.com/questions/25528/cubic-root-of-negative-numbers

Ruby is giving one of the imaginary roots, so not technically a bug, however most people probably want -2 in this situation.

Answer 12

If only you'd been here earlier Gordie. Fantastic answer ☺

Answer 13

I want to expand my previous question and ask about calculating any fractional powers of negative numbers? I mean only real roots. What rules or techniques are used to solve this problem?
Quite another question: is there a function in the Math library for calculating a fractional factorial?
Why do I continue to ask these questions because I have written the app (https://code.sololearn.com/c48HHyY3UZL2) for calculating large and nested arithmetic expressions and it has two problems described above.

Answer 14

Yes, imaginary numbers are printed, but they are incorrect. In my code this is shown.

Answer 15

Is it possible (in Ruby) to represent a decimal fraction in the form of a simple fraction? That is, for example, the number 0.2 to represent as 1/5? And if so, how to do it?