On Wed, Jul 03, 2013 at 07:56:28PM +0200, Marco Leise wrote:
> Am Tue, 2 Jul 2013 22:21:52 -0700
> schrieb "H. S. Teoh" <hsteoh@quickfur.ath.cx>:
> 
> > On Tue, Jul 02, 2013 at 10:14:33PM -0700, Ali Çehreli wrote: [...]
> > > import std.stdio;
> > > import std.conv;
> > > 
> > > void main()
> > > {
> > >     auto a = to!double("151.42499");
> > >     writefln("%.60f", a);
> > > }
> > 
> > I wouldn't write it like that; IMO it's better to write:
> > 
> > 	writefln("%.*f", double.dig, a);
> > 
> > So that you don't give the wrong impression that there are more digits than are actually there. Using double.dig also lets you see all the digits that *are* there, not a rounded value, that the OP was complaining about.
> > 
> > 
> > T
> > 
> 
> *f ? I didn't know that you can format the format string like that. Thanks for the tip.
[...]

Basically, it's a wildcard precision. Usually you'd write %.5f for 5 digits following the decimal point, for example, but that requires hardcoding the integer literal into the format string. In some cases you need it to be variable, so '*' was introduced as a stand-in for the next item in the argument list. So writefln("%.*f", 5, a) is equivalent to writefln("%.5f", a).

The wildcard * can also be used in the field-width, so writefln("%*.*f",
5, 2, 1) is equivalent to writefln("%5.2f", 1).

This isn't D's innovation, BTW, the * width/precision specifier is defined in the same way in C's printf().


T

-- 
He who does not appreciate the beauty of language is not worthy to bemoan its flaws.