math: math.Floor bugged on products of certain float64 numbers #41283
Comments
package main
import (
"fmt"
)
func main() {
var a, b float64 = 680, 0.35
fmt.Println(a, b, a*b)
}it prints So I think 237 is correct you can see https://en.wikipedia.org/wiki/IEEE_754 |
|
680.0 * 0.35 = 680 * 35 / 100 = 23800 / 100 = 238. The floor result is correct. On Ruby, this multiplication gives the incorrect result of 237.99999999999997. I'm impressed that Go manages to get this right. But still, don't use floating point for exact calculations, such as calculations involving money. EDIT: https://play.golang.org/p/LXD5hDtObtD package main
import (
"fmt"
"math"
)
func main() {
var a float64 = 680.0
var b float64 = 0.35
res := a * b
fmt.Printf("%f, %f, %v %v %v\n",
res,
math.Floor(res),
(res == 237.99999999999996),
(res > 237.99999999999995),
(res < 238.0),
)
}Prints |
|
This is not a bug. 0.35 is not exactly representable as a floating point number, so there will be rounding error. If that error is downwards, then taking the floor gets you the integer almost 1 lower than the product. |
What version of Go are you using (
go version)?Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (
go env)?go envOutputWhat did you do?
https://play.golang.org/p/jk8bVwTo3HQ
What did you expect to see?
A correct floor results.
What did you see instead?
When I tried to get the floor of 680.0*0.35, I got 237 instead of 238.
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