Arggh Golang does not include a round function in the standard math package. So I wrote a quick one.

round.go

package main

import (
	"log"
	"math"
)

func Round(val float64, roundOn float64, places int ) (newVal float64) {
	var round float64
	pow := math.Pow(10, float64(places))
	digit := pow * val
	_, div := math.Modf(digit)
	if div >= roundOn {
		round = math.Ceil(digit)
	} else {
		round = math.Floor(digit)
	}
	newVal = round / pow
	return
}

func main() {
	log.Println(Round(123.555555, .5, 3))
	log.Println(Round(123.558, .5, 2))
}

Note: this implementation fails for Round(-3.333, 0.5, 2) -- expected -3.33, got -3.34

Simple \sgn(y) \left\lfloor \left| y \right| + 0.5 \right\rfloor would be enough.

After reading through many different and untested solutions, I decided to put together a package with implementations of ToEven (used in .Net, Python 3) rounding and AwayFromZero (used in Python 2) rounding.

It includes unit tests that demonstrate the expected behaviour of the implementation, and a comparison test with Python and a rough benchmark.

See https://github.com/a-h/round

Happy to take a pull request for ToPositiveInfinity (used by Java) and ToNegativeInfinity rounding if anyone cares about them.

hi guys, I never saw these comments until just now. The implementation was old and was something quick and dirty. Thus the code is in a gist snippet and not a repo. This worked for my case which was positive integers, but note it will fail with negative numbers. It looks like people have other implementations though they don't account for what decimal place you want to round on or what value you want to round on (.5 vs .9 or other use cases.

@korya +1

https://www.carlyzach.com/functions/playground/10308388

2018! -> https://golang.org/pkg/math/#Round