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Float64 subtraction does not work for sufficiently large Floats #50328

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kevinburchfield opened this issue Dec 23, 2021 · 1 comment
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@kevinburchfield
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What version of Go are you using (go version)?

$ go version
go version go1.17.5 darwin/amd64

Reproducible in go.dev REPL

Does this issue reproduce with the latest release?

Yes

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

go env Output
$ go env

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/kb/Library/Caches/go-build"
GOENV="/Users/kb/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/kb/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/kb/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.17.5"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/kb/src/Go/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/53/0s5pcxwj3bq32v_y97xdvvrr0000gn/T/go-build2232257766=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

While implementing polynomial remainders for primality testing, I discovered the subtraction operator does not work for sufficiently large Float64.

Link to replication: https://go.dev/play/p/ET1J7tyoJTe

What did you expect to see?

I expect to see the right operand to be subtracted from the left operand

What did you see instead?

No subtraction took place.

@randall77
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This is how floating point numbers work. The epsilon near MaxFloat64 is huge, around 10^292. Subtracting values much smaller than that will return the same value you subtracted from.

Add this to your code:

fmt.Printf("%f\n", math.Log10(max-math.Nextafter(max, 0)))

@golang golang locked and limited conversation to collaborators Dec 23, 2022
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