Hi Jochen,
I think you can get what you want simply with the fmt package.
Consider fmt.Sprintf("%0.5f", x), for example, if you wanted 5 decimal
places.
If you need to actually round a floating point number, rather than just
displaying
a certain number of places, the CockroachDB guys wrote a good summary of
the approaches here, back in 2017:
https://www.cockroachlabs.com/blog/rounding-implementations-in-go/
As they mention, there is now https://pkg.go.dev/math#Round in the standard
library, which would typically be used by:
multiplying by a power of 10, math.Round()-ing, and then dividing by
the same power of 10.
On Wednesday, August 13, 2025 at 2:42:18 PM UTC+1 robert engels wrote:
> 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 <[email protected]> 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
>>
>> On Aug 13, 2025, at 7:42 AM, Jochen Voss <[email protected]> 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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