Floating point "g" format not stripping trailing zeros

[Grant Edwards]

On 2015-02-13, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Fri, Feb 13, 2015 at 7:02 AM, Grant Edwards <invalid at invalid.invalid> wrote:
>> On 2015-02-12, Ian Kelly <ian.g.kelly at gmail.com> wrote:
>>> On Thu, Feb 12, 2015 at 1:23 PM, Hrvoje Nikšić <hniksic at gmail.com> wrote:
>>>
>>>> {:.15g} is supposed to give 15 digits of precision, but with trailing
>>>> zeros removed.
>>>
>>> The doc says with "insignificant" trailing zeros removed, not all
>>> trailing zeros.
>>
>> Can somebody explain the difference between "significant" and
>> "insignificant" tailing zeros to somebody who barely passed his single
>> numerical methods class?  [Though I have, on occasion, had to tinker
>> with the innards of SW floating point libraries and could fish a
>> hardcopy of IEEE-754 out of a filing cabinet if needed.]
>
> Significant digits are within the precision of the calculation.
> Writing 1.230 indicates that the fourth digit is known to be zero.
> Writing 1.23 outside a context of exact calculation indicates that the
> fourth digit is unknown due to insufficient precision.

I knew that, but I was asking in the context of float/decimal's
formatting function.  I didn't realize that float and/or decimal had a
"significant digit" property, and therefore possess significant vs.
insignificant trailing zeros when represented in base-10.

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