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math: wrong result for Pow(±0, .5) contrary to documentation #23224

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majnemer opened this issue Dec 22, 2017 · 4 comments
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math: wrong result for Pow(±0, .5) contrary to documentation #23224

majnemer opened this issue Dec 22, 2017 · 4 comments
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Documentation FrozenDueToAge
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Go1.11

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@majnemer
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Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 linux/amd64

Does this issue reproduce with the latest release?

I believe go1.9.2 is the latest release.

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/usr/local/google/home/majnemer/go"
GORACE=""
GOROOT="/usr/lib/google-golang"
GOTOOLDIR="/usr/lib/google-golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build846653371=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

I ran the following program: https://play.golang.org/p/_lUAaRG9D-P

What did you expect to see?

The documentation for math.Pow says the following:

Pow(±0, y) = +0 for finite y > 0 and not an odd integer

.5 is finite and not an odd integer which means math.Pow should give back +0.

What did you see instead?

math.Pow gave back -0.
@majnemer majnemer changed the title Go gives wrong results for math.Pow(±0, .5) Go gives wrong result for math.Pow(±0, .5) Dec 22, 2017
@majnemer majnemer changed the title Go gives wrong result for math.Pow(±0, .5) math: wrong result for math.Pow(±0, .5) Dec 22, 2017
@gopherbot
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Change https://golang.org/cl/85415 mentions this issue: math: Fixed wrong result for sqrt(-0)

@odeke-em odeke-em changed the title math: wrong result for math.Pow(±0, .5) math: wrong result for Pow(±0, .5) contrary to documentation Dec 23, 2017
@odeke-em
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Thank you for reporting this issue @majnemer, I'll loop in @griesemer.

@griesemer griesemer added this to the Go1.11 milestone Dec 24, 2017
@griesemer griesemer self-assigned this Dec 24, 2017
@majnemer
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There is another, related bug.
Consider: Pow(-Inf, 0.5) == Pow(-0, -0.5) == Pow(±0, y) = +Inf

However, Go gives me back NaN.

I believe the original bug and the one I have just noted if we change the line:

return Sqrt(x)

into something along the lines of the following:

if IsInf(x, -1) {
  return Inf(1)
}
return Abs(Sqrt(x))

@gopherbot
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Change https://golang.org/cl/85660 mentions this issue: math: correct result for Pow(x, ±.5)

@golang golang locked and limited conversation to collaborators Jan 2, 2019
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@majnemer @odeke-em @griesemer @gopherbot