formatting a float or double in a string with all significant digits kept

[berni44]

On Tuesday, 8 October 2019 at 20:37:03 UTC, dan wrote:
> But i would like to be able to do this without knowing the 
> expansion of pi, or writing too much code, especially if 
> there's some d function like writeAllDigits or something 
> similar.

You can use the property .dig to get the number of significant 
digits of a number:

writeln(PI.dig); // => 18

You still need to account for the numbers before the dot. If 
you're happy with scientific notation you can do:

auto t = format("%.*e", PI.dig, PI);
writeln("PI = ",t);

(By the way, you can shortcut that with writefln!"PI = 
%.*e"(PI.dig, PI);)

If you don't want to use scientific notation, you probably need 
to do some calculations to find out about the number of digits 
before the dot (you need abs to make it work for negative numbers 
too):

import std.conv: to;
auto x = to!int(log10(abs(PI)));
writefln!"%*.*f"(x,PI.dig-x,PI);