floating point value rounded to 6digits

[Jonathan M Davis]

On Tuesday, September 19, 2017 19:35:15 Steven Schveighoffer via 
Digitalmars-d-learn wrote:
> On 9/19/17 7:28 PM, Ivan Kazmenko wrote:
> > On Tuesday, 19 September 2017 at 22:44:06 UTC, greatsam4sure wrote:
> >> On Tuesday, 19 September 2017 at 21:52:57 UTC, Ivan Kazmenko wrote:
> >>> On Tuesday, 19 September 2017 at 20:47:02 UTC, greatsam4sure wrote:
> >>>> double  value = 20.89766554373733;
> >>>> writeln(value);
> >>>> //Output =20.8977
> >>>>
> >>>> How do I output the whole value without using writfln,write or
> >>>> format. How do I change this default
> >>>
> >>> The default when printing floating-point numbers is to show six most
> >>> significant digits.
> >>> You can specify the formatting manually with writefln, for example,
> >>>
> >>>     writefln ("%.10f", value);
> >>>
> >>> will print the value with 10 digits after the decimal point.
> >>> The writef/writefln function behaves much like printf in C.
> >>>
> >>> See here for a reference on format strings:
> >>> https://dlang.org/library/std/format/formatted_write.html#format-strin
> >>> g
> >>>
> >>> Ivan Kazmenko.
> >>
> >> I don't  want to use write,writefln or format. I just want to change
> >> the default
> >
> > Unlikely to be possible.  The built-in data types, such as float or
> > double, by definition should not be customizable to such degree.
> >
> > Anyway, under the hood, write uses format with the default format
> > specifier "%s" for the values it takes.  So perhaps I'm not quite
> > getting what exactly are you seeking to avoid.
>
> What he's looking for is a way to globally set "I want all floating
> point values to print this way, unless a more specific specifier is
> given."
>
> It's not a terrible idea, as any code that's using %s most of the time
> doesn't really care what the result looks like.
>
> I imagine an API like this:
>
> import std.format: setDefaultFormat;
> setDefaultFormat!float("%.10f");

The big problem with that is that it does not play nicely at all with pure.
For writeln, that doesn't matter much, since it can't be pure anyway,
because it's doing I/O, but it would matter for stuff like format or
formattedWrite, which IIRC, writeln uses internally.

If what the OP wants is to change what writeln does for floating point
values, the easiest way would be for them to create their own writeln and
use that instead. Then, it can forward to std.stdio.writeln for everything
but floating point values, and for floating point values, it can call
writefln with whatever format specifier gives the desired number of decimal
places.

- Jonathan M Davis