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
<golang-nuts@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
> <reng...@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 <jochen.v...@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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