bug#25557: Documentation of format doesn't describe "g" accurately

[Clément Pit--Claudel]

On 2017-01-28 11:01, Eli Zaretskii wrote:
>> Cc: 25557@debbugs.gnu.org
>> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
>> Date: Sat, 28 Jan 2017 10:45:09 -0500
>>
>>>> I'd like something like this (with a hypothetical %q):
>>>>
>>>>   (format "%.3q" 3) ⇒ "3"
>>>>   (format "%.3q" 3.00) ⇒ "3"
>>>>   (format "%.3q" 3.30) ⇒ "3.3"
>>>>   (format "%.3q" 3.05) ⇒ "3.05"
>>>>   (format "%.3q" 3.352) ⇒ "3.35"
>>>>   (format "%.3q" 3100000) ⇒ "3100000"
>>>>
>>>> This is in fact just the same as 'g', except for the last entry (%g 
>>>> produces "3.1e+06"). Is this achievable?
>>>
>>> Yes, if you use "%.7g".  In general, use "%.Ng" if you want up to N
>>> digits in the printed representation.
>>>
>>> Does that answer your question?
>>
>> Almost: though this works for the last example, it breaks the one before the 
>> last (I'm looking for a format specifier that would constrain the number of 
>> decimals, not the number of digits, so that (format "%.3q" 30.352) would 
>> produce "30.35" — maybe all these examples should have had %.2q instead of 
>> %.3q, in fact).
> 
> Then maybe you want %.2f?

No, that always prints decimals.

> If not, then perhaps you should explain what are you trying to
> accomplish?  Why is it important to see 3.35 when the value is 3.352,
> but see all the trailing zeros for 3100000?  It sounds like a
> contradiction to me.

I'm trying to get the shortest representation of a decimal number, rounded to 
.01, not using scientific/exponential notation.  I'm using values computed in 
ELisp to produce CSS style sheets, which until recently didn't allow for 
exponential notation.  I'd like these stylesheets to be readable, so 100% is 
better 100.00%, and 3.35em is better than 3.35004em. If my code is fed a value 
of 5000px, I don't want it converted to 5e+3px, because many browsers don't 
know how to parse that.

Thanks!
Clément.

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