encoding/json 314line isEmptyValue() when float64 compares with 0, '==' is likely to be false #28111

fwhezfwhez · 2018-10-10T03:36:31Z

What did you do?

I've ever faced a float comparing with 0 problem.When I did:

func f(arg interface{}){
    switch v:=arg.(type) {
    case float32,float64:
        fmt.Println(v==0)
    }
}
func main(){
    f(0.)
}

The result turns out to be false.
I fix it by

	case float32:
		r := float64(v)
		fmt.Println(math.Abs(r-0) < 0.0000001)
	case float64:
		fmt.Println(math.Abs(v-0) < 0.0000001)

OK,Things above is not what I want to ask.The point is, I see this part in encoding/json 314 line

func isEmptyValue(v reflect.Value) bool {
	switch v.Kind() {
	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
		return v.Len() == 0
	case reflect.Bool:
		return !v.Bool()
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return v.Int() == 0
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		return v.Uint() == 0
	case reflect.Float32, reflect.Float64:
		return v.Float() == 0
	case reflect.Interface, reflect.Ptr:
		return v.IsNil()
	}
	return false
}

should it be modified as

return math.Abs(v.Float()-0)<0.0000001

?

What did you expect to see?

What did you see instead?

System details

go version go1.11 windows/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="C:\Users\DELL\AppData\Local\go-build"
GOEXE=".exe"
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="windows"
GOOS="windows"
GOPATH="G:\go_workspace\GOPATH;"
GOPROXY=""
GORACE=""
GOROOT="E:\go_1.11"
GOTMPDIR=""
GOTOOLDIR="E:\go_1.11\pkg\tool\windows_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.11 windows/amd64
GOROOT/bin/go tool compile -V: compile version go1.11
gdb --version: GNU gdb (GDB) 7.8

The text was updated successfully, but these errors were encountered:

elagergren-spideroak · 2018-10-11T06:26:08Z

The reason you're seeing an issue here is using two types in a case makes the type interface{}, not either float32 or float64 depending on the context.

From the spec:

Given an expression x of type interface{}, the following type switch:

switch i := x.(type) {
case nil:
	printString("x is nil")                // type of i is type of x (interface{})
case int:
	printInt(i)                            // type of i is int
case float64:
	printFloat64(i)                        // type of i is float64
case func(int) float64:
	printFunction(i)                       // type of i is func(int) float64
case bool, string:
	printString("type is bool or string")  // type of i is type of x (interface{})
default:
	printString("don't know the type")     // type of i is type of x (interface{})
}

You're actually comparing interface{float64(0)} to int(0), which is obviously false since the types differ. Your 'fix' can actually just compare both separate cases to 0.

isEmptyValue isn't broken because Float64 returns a float64, not interface{T}.

For example: https://play.golang.org/p/LJHI3IpzvwT

fwhezfwhez · 2018-10-11T07:00:13Z

ok,I see.
Thanks!

fwhezfwhez closed this as completed Oct 11, 2018

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encoding/json 314line isEmptyValue() when float64 compares with 0, '==' is likely to be false #28111

encoding/json 314line isEmptyValue() when float64 compares with 0, '==' is likely to be false #28111

fwhezfwhez commented Oct 10, 2018 •

edited

elagergren-spideroak commented Oct 11, 2018 •

edited

fwhezfwhez commented Oct 11, 2018

encoding/json 314line isEmptyValue() when float64 compares with 0, '==' is likely to be false #28111

encoding/json 314line isEmptyValue() when float64 compares with 0, '==' is likely to be false #28111

Comments

fwhezfwhez commented Oct 10, 2018 • edited

What did you do?

What did you expect to see?

What did you see instead?

System details

elagergren-spideroak commented Oct 11, 2018 • edited

fwhezfwhez commented Oct 11, 2018

fwhezfwhez commented Oct 10, 2018 •

edited

elagergren-spideroak commented Oct 11, 2018 •

edited