[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Change by Mark Dickinson :


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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

All done; closing. Thank you for the contribution!

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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:


New changeset ac867f10b49322e25f34d2d8abd8e63c86834750 by Ajith Ramachandran in 
branch 'main':
bpo-44357:Add `math.cbrt()` function: Cube Root (GH-26622)
https://github.com/python/cpython/commit/ac867f10b49322e25f34d2d8abd8e63c86834750


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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

If we *really* wanted to bikeshed on the name, back in 1991 Kahan wrote:

> Perhaps the last problem is the hardest: choosing the program's name. Ideally 
> it should need no explanation, but a limitation upon its length may preclude 
> that. Although "CBRT" has seen use, I prefer "QBRT" in order that the prefix 
> "C" may be reserved for use with complex-valued functions.

Source: https://csclub.uwaterloo.ca/~pbarfuss/qbrt.pdf

But that was 30 years ago, and I think the "CBRT"-shaped ship has long since 
sailed.

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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

@Victor: Yep, the name is pretty standard. Not just C, but JavaScript, Java, R, 
and likely a lot of other languages that I haven't checked.

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[issue44357] Add math.cbrt() function: Cube Root

[STINNER Victor]

STINNER Victor  added the comment:

I didn't know the "cbrt" function name. It seems like it exists in the libc, 
but also in numpy. So it's a good idea to reuse this name ;-)

numpy.cbrt(): Return the cube-root of an array, element-wise.
https://numpy.org/doc/stable/reference/generated/numpy.cbrt.html

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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

There's also a decent chance that a libm implementation of cbrt will be 
correctly rounded, while `x**(1/3)` is highly unlikely to be so.

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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

> Perhaps because it is so easy to write x**(1/3), and if you want a real root 
> of negative argument, it is -(-x)**(1/3).

I consider `x**(1/3)` to be a trap and a bug magnet: many people won't realise 
that the correct spelling for their situation is actually `def f(x): return 
-((-x)**(1/3)) if x < 0 else x**(1/3)` and the failure mode of `x**(1/3)` for 
negative `x` seems suboptimal, in that it happily returns a complex number 
rather than raising a `ValueError` with a clear message indicating what was 
wrong. I've seen a good number of Stack Overflow questions from users confused 
about this exact thing.

And not only is that correct spelling unwieldy, but it _still_ doesn't do the 
right thing for `-0.0`, and if you care about getting that corner case right 
then the right spelling becomes even more unwieldy.

Of course, even with `math.cbrt` in the standard library people will still fall 
into the `x**(1/3)` trap, but at least we then have a decent replacement to 
point them to.

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[issue44357] Add math.cbrt() function: Cube Root

[Ajith Ramachandran]

Ajith Ramachandran  added the comment:

Yes I was aware of x**(1/3) and used that. I just thought it would be useful if 
it was present in math module as it is also used in many situations like 
`sqrt()`.

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[issue44357] Add math.cbrt() function: Cube Root

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I am surprised that this was not proposed before. Perhaps because it is so easy 
to write x**(1/3), and if you want a real root of negative argument, it is 
-(-x)**(1/3). It can be slightly less accurate that the result of C library 
function cbrt(), but good enough for any practical applications.

Ajith, was you aware of x**(1/3)?

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[issue44357] Add math.cbrt() function: Cube Root

[Ajith Ramachandran]

Ajith Ramachandran  added the comment:

When I needed a function to find cube root, I couldn't find one in the math 
module. Also found the `cbrt()` function in C. I didn't account for the complex 
numbers. I didn't wanted to make it a complex function unnecessarily. So I used 
the `cbrt()` function directly and it was working well for my test cases. And 
it is supported by visual studio.

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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

BTW, to forestall questions about adding cmath.cbrt: it's not obvious how to 
extend the real cube root function to a complex cube root, so it might make 
sense to wait for actual use-cases before doing so.

The issue is that the most natural way to define a complex cube root would use 
a branch cut along the negative real axis (the same branch cut that cmath.sqrt 
and cmath.log use). But then the principal branch would *not* be an extension 
of math.cbrt: it would return non-real values on the negative real axis. (The 
*real* cube roots of negative numbers would fall on the two non-principal 
branches.)

See for example 
https://math.stackexchange.com/questions/71775/extending-the-cube-root-function-to-mathbbc

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[issue44357] Add math.cbrt() function: Cube Root

[Mark Dickinson]

Mark Dickinson  added the comment:

+1. This is part of C99, so if it's also supported by Visual Studio, then this 
seems like a no-brainer. If it's _not_ also supported by Visual Studio, or if 
there are implementations that have serious numerical problems (as was the case 
with fma) we'll need to write our own fallback implementation, which is less 
simple.

In principle it should be relatively easy for a math library to provide a 
correctly-rounded cbrt.

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[issue44357] Add math.cbrt() function: Cube Root

[Ajith Ramachandran]

Change by Ajith Ramachandran :


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keywords: +patch
pull_requests: +25206
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/26622

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[issue44357] Add math.cbrt() function: Cube Root

[Serhiy Storchaka]

New submission from Serhiy Storchaka :

See also issue27353.

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nosy: +lemburg, mark.dickinson, rhettinger, serhiy.storchaka, stutzbach
versions: +Python 3.11 -Python 3.10

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[issue44357] Add math.cbrt() function: Cube Root

[Ajith Ramachandran]

Change by Ajith Ramachandran :


--
components: Library (Lib)
nosy: AjithRamachandran
priority: normal
severity: normal
status: open
title: Add math.cbrt() function: Cube Root
type: enhancement
versions: Python 3.10

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