A floating point number cannot represent all possible real numbers - so when you “shift it back” the number you expect may not be possible.
See this on why 0.1 cannot be represented. https://how.dev/answers/why-does-01-not-exist-in-floating-point > On Aug 13, 2025, at 7:55 AM, Jochen Voss <jochen.v...@gmail.com> wrote: > > Dear Robert, > > Thank you for your response. To make sure I understand: Are you saying > Claude's Round() function does not work? (For which example?) Or are you > saying it is impossible to do better than Claude's function? > > Many thanks, > Jochen > > On Wednesday, 13 August 2025 at 13:52:08 UTC+1 robert engels wrote: >> 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 <http://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 >>> >>> >>> >> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "golang-nuts" group. >>> To unsubscribe from this group and stop receiving emails from it, send an >>> email to golang-nuts...@googlegroups.com <>. >>> To view this discussion visit >>> https://groups.google.com/d/msgid/golang-nuts/7d3ccdd4-d88b-4eba-8a36-02b51b7751e1n%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/golang-nuts/7d3ccdd4-d88b-4eba-8a36-02b51b7751e1n%40googlegroups.com?utm_medium=email&utm_source=footer>. >> > > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com > <mailto:golang-nuts+unsubscr...@googlegroups.com>. > To view this discussion visit > https://groups.google.com/d/msgid/golang-nuts/1e3872a4-6b05-4f03-9dd3-37d1b6955297n%40googlegroups.com > > <https://groups.google.com/d/msgid/golang-nuts/1e3872a4-6b05-4f03-9dd3-37d1b6955297n%40googlegroups.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/1CCEDE74-345C-4E9F-97FB-87316AB45747%40ix.netcom.com.
