R devel - Aug 2018 - RFC: make as.difftime more consistent or convenient

Hello!

you, Emil Bode <emil.bode at dans.knaw.nl>, wrote on Tuesday, July 31,
2018 1:55 PM:
> Some of the changes you're proposing could be made (with effort), but
note that you're not
> restricted to providing strings with a format.
> What you're trying to do can be accomplished with as.difftime(12,
units='weeks'), see also
> ?as.difftime
>
> Or if you're stuck with the strings:
as.difftime(as.numeric(substring('12 w', 1, 2)),
> units='weeks')
> That also seems clearer, because in your script, the last part of your
string simply gets
> ignored: as.difftime("12 h", "%H") reads your string
for something that satisfies %H (which
> 12 does), then stops:
> as.difftime("12 hours and 17 minutes", "%H") gives 12
hours. If you wanted to check, you
> could have used as.difftime("12 h", "%H h")
Thank you for your comments! But, what you wrote is known. What do you want to
express with regard to my questions?

I wrote:
> ? there is no appropriate format ...,
> although "weeks" is a legitimate unit of 'difftime':
>
>     > as.difftime("12 w", "%...")
>
>     1. What do you think about making the behavior of 'as.difftime'
more consistent by
> accepting also formats for "days" and "weeks"?
>
>     2. Even more convenient it could be if 'as.difftime' accepted
strings containing
> magnitude and unit; how about:
>
>     > as.difftime("12 weeks")
>     Time difference of 12 weeks
I'm asking for comments in order to get feedback whether it would be
well-received if I proposed or provided the former or the latter change on
Bugzilla.
--
Best regards,
Dietmar Schindler
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> Thank you for your comments! But, what you wrote is known. What do you
> want to express with regard to my questions?
> 
> I wrote:
> > ? there is no appropriate format ...,
> > although "weeks" is a legitimate unit of 'difftime':
> >
> >     > as.difftime("12 w", "%...")
> >
> >     > as.difftime("12 weeks")
> >     Time difference of 12 weeks
> >
> >     1. What do you think about making the behavior of
'as.difftime' more
> >   consistent by accepting also formats for "days" and
"weeks"?
as.difftime calls strptime to apply the format argument.

If I  wanted to extend the range of formats as.difftime accepts, I'd leave
as.difftime alone and look at how strptime could be extended to cover the
formats you envisage.

But... I wouldn?t do that either. strptime is essentially a call to an .Internal
function and very likely reliant on established  C code for the already very
flexible standard C function strptime, which strptime clearly mirrors
intentionally. That usually makes things dangerous to tinker with in the short
term and hard to maintain in the long term.

So  if you want to do something that will readily convert all combinations of
things like '12 w', '12W', '12wks', '3m 2d', 1wk
2d', '18d' etc, write that as a stand-alone routine that converts
those 'simple' formats directly to difftime objects and call it
something like 'strpdifftime', which would allow it to be added (if
it's wanted a lot) with minimal impact to existing code.

S Ellison




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