Go version

go version go1.23.4 windows/amd64

Output of go env in your module/workspace:

set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=D:\Enviornment\GO\src\cache
set GOENV=C:\Users\admin\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=D:\Workspace\GO\pkg\mod
set GONOPROXY=gitee.com
set GONOSUMDB=gitee.com
set GOOS=windows
set GOPATH=D:\Workspace\GO
set GOPRIVATE=gitee.com
set GOPROXY=https://goproxy.cn,direct
set GOROOT=D:\Enviornment\GO
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=D:\Enviornment\GO\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.23.4
set GODEBUG=
set GOTELEMETRY=local
set GOTELEMETRYDIR=C:\Users\admin\AppData\Roaming\go\telemetry
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=D:\Workspace\GO\src\gitlab\readboy\wrong-question-book\go.mod
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\admin\AppData\Local\Temp\go-build2290248184=/tmp/go-build -gno-record-gcc-switches

What did you do?

package utils_test

import (
    "fmt"
    "math"
    "testing"
)

func TestToPercentage(t *testing.T) {
    testcases := []struct {
        value float64
        digit int
        want  string
    }{
        // {0.123456, 2, "12.35%"},
        // {0, 0, "0"},
        // {0.123456, 4, "12.3456%"},
        // {0.5, 0, "50%"},
        // {0.5, 1, "50%"},
        // {1, 0, "100%"},
        {0.5666666666666667, 0, "57%"},
    }
    for idx, tc := range testcases {
        got1 := ToPercentage(tc.value, tc.digit)
        got2 := ToPercentage2(tc.value, tc.digit)
        if got1 != got2 {
            t.Errorf("%d. ToPercentage(%v) = %s, value1: %s, value2: %s", idx, tc.value, tc.want, got1, got2)
        }
    }
}

func ToPercentage(value float64, n int) string {
    if value == 0 {
        return "0"
    }
    r2 := fmt.Sprintf("%v%%", math.Round(value*math.Pow10(n+2))/math.Pow10(n))
    return r2
}

func ToPercentage2(value float64, n int) string {
    if value == 0 {
        return "0"
    }
    return fmt.Sprintf("%v%%", Round(value, n+2)*100)
}

func Round(value float64, digit int) float64 {
    return math.Round(value*math.Pow10(digit)) / math.Pow10(digit)
}

What did you see happen?

--- FAIL: TestToPercentage (0.00s)
    d:\Workspace\GO\src\gitlab\readboy\wrong-question-book\utils\ratio_test.go:27: 0. ToPercentage(0.5666666666666667) = 57%, value1: 57%, value2: 56.99999999999999%
FAIL
FAIL    wrong-question-book/utils   0.652s
FAIL

What did you expect to see?

the testcase is correct

Comment From: seankhliao

I believe this is correct due to how floating point represents numbers.

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions