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Inconsistent rounding of floating point #34815

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jeetsang opened this issue Oct 10, 2019 · 1 comment
Closed

Inconsistent rounding of floating point #34815

jeetsang opened this issue Oct 10, 2019 · 1 comment
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@jeetsang
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jeetsang commented Oct 10, 2019 •
edited by ALTree

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

$ go version go1.12.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env
$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jitendra/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jitendra/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/x8/68sd4h_x0v140qx5jb18mp5c0000gn/T/go-build192220704=/tmp/go-build -gno-record-gcc-switches -fno-common

What did you do?

fmt.Printf("%.1f", 3.85)
fmt.Println()
fmt.Printf("%.1f", 4.85)

What did you expect to see?

3.9

4.9

What did you see instead?

3.9

4.8
@ALTree
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ALTree commented Oct 10, 2019 •
edited

The rounding is correct. Neither 3.85 nor 4.85 are exactly representable as IEEE-754 floats, so even if you wrote "3.85" and "4.85" in your code, what you'll really get is the nearest representable float64s, which are:

3.850000000000000088817841970012523233890533447265625
4.8499999999999996447286321199499070644378662109375

which explains why the first is rounded to .9 and the second to .8

Closing here, since this is working as intended.

@ALTree ALTree closed this as completed Oct 10, 2019
@golang golang locked and limited conversation to collaborators Oct 9, 2020
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@jeetsang @ALTree @gopherbot