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catalase opened this issue Oct 25, 2016 · 6 comments
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math/cmplx: Cot(0) loops infinitely #17577

catalase opened this issue Oct 25, 2016 · 6 comments
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@catalase
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catalase commented Oct 25, 2016 •
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Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go1.7 (I tested on play.golang.org)

What did you do?

  1. https://play.golang.org/p/TYwrN6agmD

cmplx.Cot(0) will loop infinitely

  1. https://play.golang.org/p/b4IG3wqBm_

let f be a function in the pakcage, and let g be the inverse function of f if f is invertible. If math/cmplx has no error in its implementation, then f(g(z)) is z for a complex z that is well-defined on f and g, which I tested.

but some function calculates wrongly, and some has poor error.
@odeke-em
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Just a drive by comment here, if I recall from my rusty Maths:

  • cot(x) = 1/tan(x) where tan(0) = 0
    coming from 1/tan(x) = cos(x)/sin(x):
    and cos(0) = 1, sin(0) = 0 so:
    cot(0) = 1/0 => Infinity.

I wonder if the package has a problem with detecting this case and should return infinity asap.
/cc @griesemer @rsc

@odeke-em odeke-em changed the title math/cmplx: cmplx.Cot(0) will loop infinitely, the cmplx package seems broken math/cmplx: Cot(0) loops infinitely Oct 25, 2016
@0xmohit
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0xmohit commented Oct 25, 2016

The issue happens due to floating-point comparison here. In this specific case, t/d is NaN and the comparison returns false resulting in an infinite loop. Will send a CL.

@gopherbot
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CL https://golang.org/cl/31952 mentions this issue.

@rakyll rakyll added this to the Go1.8 milestone Oct 25, 2016
gopherbot pushed a commit that referenced this issue Oct 25, 2016
@0xmohit @griesemer
math/cmplx: prevent infinite loop in tanSeries
5a95492 
The condition to determine if any further iterations are needed is
evaluated to false in case it encounters a NaN. Instead, flip the
condition to keep looping until the factor is greater than the machine
roundoff error.

Updates #17577

Change-Id: I058abe73fcd49d3ae4e2f7b33020437cc8f290c3
Reviewed-on: https://go-review.googlesource.com/31952
Reviewed-by: Robert Griesemer <gri@golang.org>
@odeke-em
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@0xmohit and @griesemer, did y'all mean to say "Fixes" instead of "Updates" with the CL https://go-review.googlesource.com/31952 that prevents an infinite loop in tanSeries, or is there more work for this issue?

@0xmohit
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0xmohit commented Oct 26, 2016

I probably specified "Updates" because more tests could be added for Cot. However, those aren't strictly related to this change.

@griesemer
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@odeke-em Thanks for heads-up. Yes, I would consider this fixed.

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