Dear Peter,
Thanks for the reference!
All the best,
Jochen
On Wednesday, 13 August 2025 at 14:10:50 UTC+1 Peter Weinberger (温博格) wrote:
> The canonical reference on this is by Guy Steele, "How to Print
> Floating Point Numbers Accurately"
>
> On Wed, Aug 13, 2025 at 9:07 AM 'Alexander Ertli' via golang-nuts
> <golan...@googlegroups.com> wrote:
> >
> > Hi Jochen,
> >
> > I think it's possible with a trick.
> >
> > My first naive thought on how to solve this is to simply shift the
> decimal point over, do the rounding on the whole number, and then shift it
> back.
> >
> > import "math"
> > // Round performs rounding by shifting the decimal, rounding, and
> shifting back.
> > func Round(x float64, digits int) float64 {
> > scale := math.Pow(10, float64(digits))
> > return math.Round(x*scale) / scale
> > }
> >
> > https://go.dev/play/p/_DVsD45FAKb
> >
> >
> > Am Mi., 13. Aug. 2025 um 14:51 Uhr schrieb robert engels <
> ren...@ix.netcom.com>:
> >>
> >> Read up on numerical analysis - what you are asking for is impossible :)
> >>
> >> You need to convert to a string, or use BCD/fixed place values - like
> github.com/robaho/fixed
> >>
> >> On Aug 13, 2025, at 7:42 AM, Jochen Voss <joche...@gmail.com> wrote:
> >>
> >> Dear all,
> >>
> >> I would like to define a function "func Round(x float64, digits int)
> float64" which rounds to the given number of digits, in the sense that I
> want "strconv.FormatFloat(x, 'f', -1, 64)" to show at most the given number
> of digits after the decimal point.
> >>
> >> The following naive approach does not work:
> >>
> >> func Round(x float64, digits int) float64 {
> >> eps := math.Pow10(-digits)
> >> return math.Round(x/eps) * eps
> >> }
> >>
> >> For example for rounding math.Pi to five digits I get
> "3.1415900000000003" instead of "3.14159".
> https://go.dev/play/p/gRtHG6ZgTjj .
> >>
> >> Anthropic's Claude suggested the following:
> >>
> >> func Round(x float64, digits int) float64 {
> >> if digits <= 0 {
> >> pow := math.Pow10(-digits)
> >> return math.Round(x/pow) * pow
> >> }
> >>
> >> format := "%." + strconv.Itoa(digits) + "f"
> >> s := fmt.Sprintf(format, x)
> >> result, _ := strconv.ParseFloat(s, 64)
> >>
> >> return result
> >> }
> >>
> >> This seems to work, but also seems quite inefficient.
> >>
> >> Is there a better way?
> >>
> >> All the best,
> >> Jochen
> >>
> >> PS.: Here is some testing code for experimenting
> https://go.dev/play/p/Xcd6fTvYend
> >>
> >>
> >>
> >> --
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> >>
> >>
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