Full precision double to string conversion

[Ecstatic Coder]

On Saturday, 3 November 2018 at 18:04:07 UTC, Stanislav Blinov 
wrote:
> On Saturday, 3 November 2018 at 17:26:19 UTC, Ecstatic Coder 
> wrote:
>
>>> void main() {
>>>     double value = -12.000123456;
>>>     int precision = 50;
>>>
>>>     import std.stdio;
>>>     writefln("%.*g", precision, value);
>>>
>>>     import std.format;
>>>     string str = format("%.*g", precision, value);
>>>     writeln(str);
>>> }
>>>
>>> Prints:
>>>
>>> -12.000123456000000743415512260980904102325439453125
>>> -12.000123456000000743415512260980904102325439453125
>>>
>>> That's not quite the -12.000123456 that you'd get from C#'s 
>>> ToString().
>>
>> Unfortunately, but that's still better though, thanks :)
>
> I don't think you understood what I meant. Neither C# nor D 
> attempt to exhaust the precision when converting, given default 
> arguments. It's merely a matter of those defaults. The snippet 
> above obviously provides *more* digits that the default 
> .ToString() in C# would.
>
>> But indeed what I really need is a D function which gives a 
>> better decimal approximation to the provided double constant, 
>> exactly in the same way those in Dart and C# do.
>>
>> Is there really no such function in D ?
>
> When you call .ToString() in C# with no arguments, it assumes 
> the "G" format specifier.
>
> https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings?view=netframework-4.7.2#the-general-g-format-specifier
>
> So for a double, it will use 15-digit precision. D's to!string 
> simply uses lower default. If you want the exact same behavior 
> as in C#, you can do this:
>
> string toStringLikeInCSharp(double value) {
>     import std.format : format;
>     return format("%.15G", value);
> }
>
> void main() {
>     double value = -12.000123456;
>     import std.stdio;
>     writeln(value.toStringLikeInCSharp); // prints: 
> -12.000123456
> }

This version perfectly gets the job done!

Thanks a lot for your help :)