GNU bug report logs - #25557
Documentation of format doesn't describe "g" accurately

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Message #10 received at 25557-done <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Clément Pit--Claudel <clement.pitclaudel <at> live.com>
Cc: 25557-done <at> debbugs.gnu.org
Subject: Re: bug#25557: Documentation of format doesn't describe "g" accurately
Date: Sat, 28 Jan 2017 10:37:25 +0200

> From: Clément Pit--Claudel <clement.pitclaudel <at> live.com>
> Date: Fri, 27 Jan 2017 17:05:28 -0500
> 
> There seems to be a few issues in the docstring of `format':
> 
> > %g means print a number in exponential notation
> >  or decimal-point notation, whichever uses fewer characters.
> 
> This seems wrong:
> 
>   (format "%g" 3.0) ⇒ "3", but
>   (format "%f" 3.0) ⇒ "3.000000", and
>   (format "%e" 3.0) ⇒ "3.000000e+00", so %g is neither %f nor %e.

Fixed.

> > The # flag means to use an alternate display form [...]
> > for %e, %f, and %g, it causes a decimal point to be included even if
> > the precision is zero.
> 
> This seems incomplete:
> 
>   (format "%#.5g" 3) ⇒ "3.0000", while
>   (format "%.5g" 3) ⇒ "3", so # doesn't just cause changes when the precision is 0.

I don't understand what you are trying to say (nor the significance of
the '.'  flag in the example).  '#' forces %g to leave the trailing
zeros after the decimal, so I added that -- is that what you wanted to
say?

> > For %e, %f, and %g sequences, the number after the "." in the
> > precision specifier says how many decimal places to show
> 
> This seems wrong, too:
> 
>   (format "%.5g" 3.0) ⇒ "3", not "3.00000"
>   (format "%.5g" 3.1) ⇒ "3.1", not "3.10000"

Fixed.

> Similar problems seem to exist in the actual documentation.

If you mean the ELisp manual, I fixed that as well.

> On a related note, is there a way to get a shortest representation of a number? Something like %g, but without exponents.

Sorry, I don't understand the question.  How can you represent an
arbitrary number without exponents at all, except by %f?

Anyway, thanks; I'm marking this bug done.

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