inverse of float-time?

inverse of float-time?

[Sam Steingold]

Is it possible to convert the return value of float-time back to a list
of integers?
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Re: inverse of float-time?

[Paul Eggert]

On 09/11/13 10:39, Sam Steingold wrote:
> Is it possible to convert the return value of float-time back to a list
> of integers?

That's not easy to do now, but it'd be easy to add.
We could add an optional argument to current-time,
decoded just like float-time's argument is decoded,
and current-time could convert it to a list of integers.

Re: inverse of float-time?

[Sam Steingold]

> * Paul Eggert <rttreg@pf.hpyn.rqh> [2013-09-11 12:14:16 -0700]:
>
> On 09/11/13 10:39, Sam Steingold wrote:
>> Is it possible to convert the return value of float-time back to a list
>> of integers?
>
> That's not easy to do now, but it'd be easy to add.

Please do!

> We could add an optional argument to current-time,
> decoded just like float-time's argument is decoded,
> and current-time could convert it to a list of integers.

Sounds good!
It would also be nice if all functions which work with lists accepted
floats too, e.g., `format-time-string'.

-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 13.04 (raring) X 11.0.11303000
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Re: inverse of float-time?

[Leo Liu]

On 2013-09-12 03:31 +0800, Sam Steingold wrote:
>> That's not easy to do now, but it'd be easy to add.
>
> Please do!

Doesn't seconds-to-time do what you want?

Leo

Re: inverse of float-time?

[SAKURAI Masashi]

Hi,

> Is it possible to convert the return value of float-time back to a list
> of integers?

I diagramed date and time conversions:

https://cacoo.com/diagrams/lsA64PTazlLTbSwR

I hope it would be useful for you.

--
SAKURAI, Masashi (family, given)
m.sakurai@kiwanami.net

Re: inverse of float-time?

[Sam Steingold]

Hi,

> * SAKURAI Masashi <z.fnxhenv@xvjnanzv.arg> [2013-09-12 12:07:15 +0900]:
>
>> Is it possible to convert the return value of float-time back to a list
>> of integers?
>
> I diagramed date and time conversions:
>
> https://cacoo.com/diagrams/lsA64PTazlLTbSwR

Interesting, thanks.
What is "howm"?

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Sam Steingold (http://sds.podval.org/) on Ubuntu 13.04 (raring) X 11.0.11303000
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Re: inverse of float-time?

[Sam Steingold]

> * Leo Liu <fqy.jro@tznvy.pbz> [2013-09-12 09:52:05 +0800]:
>
> On 2013-09-12 03:31 +0800, Sam Steingold wrote:
>>> That's not easy to do now, but it'd be easy to add.
>>
>> Please do!
>
> Doesn't seconds-to-time do what you want?

Ah, yes, it does. Thanks!

However, what I want is for float time to be transparently accepted by
all functions which accept list time.

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Sam Steingold (http://sds.podval.org/) on Ubuntu 13.04 (raring) X 11.0.11303000
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Re: inverse of float-time?

[Paul Eggert]

Sam Steingold wrote:
> what I want is for float time to be transparently accepted by
> all functions which accept list time.

On second thought I'm not sure I want to encourage that.
Floating-point loses information, due to rounding, and
this can cause glitches that can make floating-point
more trouble than it's worth.

For example, if gettimeofday reports 1378930161.099999786 seconds
after 1970, then current-time returns the exact representation
(21040 52721 99999 786000) but float-time rounds this to
1378930161.1.  Sure, it's a small error, but the fact that
it's nonzero can cause problems.

Re: inverse of float-time?

[Ulrich Mueller]

>>>>> On Wed, 11 Sep 2013, Paul Eggert wrote:

> Sam Steingold wrote:
>> what I want is for float time to be transparently accepted by
>> all functions which accept list time.

> On second thought I'm not sure I want to encourage that.
> Floating-point loses information, due to rounding, and this can
> cause glitches that can make floating-point more trouble than it's
> worth.

> For example, if gettimeofday reports 1378930161.099999786 seconds
> after 1970, then current-time returns the exact representation
> (21040 52721 99999 786000) but float-time rounds this to
> 1378930161.1.  Sure, it's a small error, but the fact that
> it's nonzero can cause problems.

Python also uses floating point numbers for time, and the error is
large enough to lose three decimal digits for filesystem timestamps
which have nanosecond precision. This is really awkward when one needs
to compare timestamps for equality.

(It's not surprising, because the IEEE 754 double format provides only
52 bits for the mantissa, whereas 31 + 30 = 61 bits are (currently!)
needed to represent seconds and nanoseconds.)

Ulrich

Re: inverse of float-time?

[Sam Steingold]

> * Paul Eggert <rttreg@pf.hpyn.rqh> [2013-09-11 21:16:22 -0700]:
>
> Sam Steingold wrote:
>> what I want is for float time to be transparently accepted by
>> all functions which accept list time.
>
> On second thought I'm not sure I want to encourage that.
> Floating-point loses information, due to rounding, and
> this can cause glitches that can make floating-point
> more trouble than it's worth.

So, what is the official use case for float time?
When one would use it instead of time-date?

It would seem that float-time should be either deprecated or granted
equal rights :-)


-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 13.04 (raring) X 11.0.11303000
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History doesn't repeat itself, but historians do repeat each other.

Re: inverse of float-time?

[Paul Eggert]

On 09/17/13 13:03, Sam Steingold wrote:

> float-time should be either deprecated or granted equal rights :-)

It should be deprecated.

float-time is based on assumptions that go back to the Good
Old Days, when time stamps typically were 32-bit signed
integer counts of seconds, possibly with an additional
microseconds count.  Under these assumptions float-time
doesn't lose information on typical platforms with IEEE
doubles, which are just barely large enough.

Nowadays, though, time_t is often 64 bits, and timestamps
are typically nanosecond resolution.  IEEE doubles can't
represent these time stamps without losing information, so
float-time generates results with rounding errors.

If we could assume Guile, I suppose we could represent these modern
time stamps exactly, using fractions.  That may be a good
way to go in the future.  It'd be a lot more convenient than
the current list-of-integers representation.

> So, what is the official use case for float-time?
> When one would use it instead of time-date?

I assume that it was put in to make it convenient to compare
or subtract time stamps, or to add a time stamp to a seconds
count.  (You added float-time back in 2000, so you should
know better than I. :-)  These days, one should use
time-less-p, time-subtract, and time-add, as they shouldn't
have rounding errors.