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runtime: Inconsistency Multiplication, Division, and Addition Result #51031

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gho1b opened this issue Feb 6, 2022 · 2 comments
Closed

runtime: Inconsistency Multiplication, Division, and Addition Result #51031

gho1b opened this issue Feb 6, 2022 · 2 comments

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@gho1b
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@gho1b gho1b commented Feb 6, 2022

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

$ go version
go version go1.17.6 windows/amd64

Does this issue reproduce with the latest release?

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

go env Output
$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\user\AppData\Local\go-build
set GOENV=C:\Users\user\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\user\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\user\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17.6
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\user\Documents\interview-codes\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\user\AppData\Local\Temp\go-build2510967983=/tmp/go-build -gno-record-gcc-switches

What did you do?

image

What did you expect to see?

I thought that c must be eqal to d

What did you see instead?

In another case i get correct results just like this
image

but the arithmetic give difference results by this formula

image

I got same issue with division and addition
image

image

image

test codes

func c1() {
a	:=	0.61	*	275.0
b	:=	0.62	*	275.0
c	:=	0.63	*	275.0
d	:=	0.64	*	275.0
e	:=	0.65	*	275.0
f	:=	0.66	*	275.0
g	:=	0.67	*	275.0
h	:=	0.68	*	275.0
i	:=	0.69	*	275.0
fmt.Printf("%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n", a, b, c, d, e, f, g, h, i)
}

func c2() {
z := 275.
a	:=	0.61	*	z
b	:=	0.62	*	z
c	:=	0.63	*	z
d	:=	0.64	*	z
e	:=	0.65	*	z
f	:=	0.66	*	z
g	:=	0.67	*	z
h	:=	0.68	*	z
i	:=	0.69	*	z
fmt.Printf("%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n", a, b, c, d, e, f, g, h, i)				
}

func c3() {
	for i := 0; i < 10; i++ {
		a := float64(i)/100 + 0.5
		fmt.Printf("i = %d; a =  %g\n", i, a)
	}
}	

func c4() {
	z := 0.5
	a := 0.01 + z
	b := 0.02 + z
	c := 0.03 + z
	d := 0.04 + z
	e := 0.05 + z
	f := 0.06 + z
	g := 0.07 + z
	h := 0.08 + z
	i := 0.09 + z
	fmt.Printf("%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n", a, b, c, d, e, f, g, h, i)
}

func c5() {
	a := 0.01 + 0.5
	b := 0.02 + 0.5
	c := 0.03 + 0.5
	d := 0.04 + 0.5
	e := 0.05 + 0.5
	f := 0.06 + 0.5
	g := 0.07 + 0.5
	h := 0.08 + 0.5
	i := 0.09 + 0.5
	fmt.Printf("%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n%g\n", a, b, c, d, e, f, g, h, i)
}
@robpike
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@robpike robpike commented Feb 6, 2022

This is how floating-point calculations work on all computers vs. how constants work in Go. They do not behave the same.

Please read these two links:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html
https://go.dev/blog/constants

@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Feb 6, 2022

Please post plain text as plain text, not as images. Images are difficult to read. Thanks.

Closing this issue because it is not a bug in Go. Please comment if you disagree.

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3 participants