math/big: (*Rat).SetString("1234") not the same as big.NewRat(1234, 1)

[damastes]

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

1.11.2

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
GOHOSTARCH="amd64"
GOHOSTOS="linux"

What did you do?

func main() {
	var x, y big.Rat
	
	x = *big.NewRat(123456789, 1)
	y.SetString("123456789")
	
	fmt.Printf("%#v\n", x)
	fmt.Printf("%#v\n", y)
}

https://play.golang.org/p/yFKM2k6djNA

What did you expect to see?

I would expect to see both structures be identical.

big.Rat{a:big.Int{neg:false, abs:big.nat{0x75bcd15}}, b:big.Int{neg:false, abs:big.nat{}}}
big.Rat{a:big.Int{neg:false, abs:big.nat{0x75bcd15}}, b:big.Int{neg:false, abs:big.nat{}}}

What did you see instead?

While the same numerical value, they are different for the purposes of test assertion.

big.Rat{a:big.Int{neg:false, abs:big.nat{0x75bcd15}}, b:big.Int{neg:false, abs:big.nat{}}}
big.Rat{a:big.Int{neg:false, abs:big.nat{0x75bcd15}}, b:big.Int{neg:false, abs:big.nat(nil)}}

[ianlancetaylor]

As a side note, you should not write x := *big.NewRat(...). Shallow copies of *big.Rat values are not supported. This will be more clearly documented in the upcoming 1.12 release (https://tip.golang.org/pkg/math/big/#Rat).

To address the main issue, you should compare *big.Rat values using the Cmp method. The package makes no promise that values that compare as equal use the same internal representation.