math.Mod seems to produce wrong output

[burdiyan]

Does this issue reproduce with the latest release?

Yes

What did you do?

package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Printf("%.8f", math.Mod(10, 0.1))
}

https://play.golang.org/p/s725Z6unzo1

What did you expect to see?

I expect 10 % 0.1 to return 0, which is the result I get calculating modulo in a spreadsheet.

What did you see instead?

I see 0.1.

[ALTree]

Thanks for reporting this. math.Mod mirrors the fmod C function in math.h, which also returns 0.1 for fmod(10, 0.1).

Your spreadsheet probably uses a different definition of floating point mod (AFAIK there are several different and conflicting definitions); but since the reference for us is the C stdlib one, 0.1 is correct and this is working as intended.

Closing here.

[rsc]

0.1 is not an exact floating point number.
It is really x = 0.1000000000000000055511151231257827021181583404541015625.
100 * x = 10.00000000000000055511151231257827021181583404541015625,
which is bigger than ten. So math.Mod returns
10.0 - 99 * x = 10.0 - 9.9000000000000005495603971894524875096976757049560546875
= .0999999999999994504396028105475124903023242950439453125,
which rounds to 0.1000000000000000055511151231257827021181583404541015625.

For more about floating point, see https://floating-point-gui.de/.