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Why can't you exactly divide a year-long interval by days? #445

hadley opened this Issue Jul 27, 2016 · 4 comments


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hadley commented Jul 27, 2016

today <- today()
next_year <- today() + years(1)

(today %--% next_year) / days(1)
#> Remainder cannot be expressed as fraction of a period.
#>  Performing %/%.
#> estimate only: convert periods to intervals for accuracy
#> [1] 365

I think the warning "convert periods to intervals for accuracy" is also spurious. What period am I supposed to convert?


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garrettgman commented Jul 27, 2016

I think this is a poorly written attempt to warn people away from / with periods (which does not have a precise definition) to %/% which has a precise definition.

/ doesn't have a precise definition because there is no clear rule about how the arithmetic operator should return the remainder in cases like this,

today <- today()
next_yearish <- today() + years(1) + days(1)

(today %--% next_yearish) / months(1)

Perhaps the message should just say something like, "Performing %/%; convert denominator to an interval or duration if you want a remainder."


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vspinu commented Jul 28, 2016

I actually started removing those messages here and there. It's more annoying than helpful. The approximate computation with periods is stated in the docs in multiple places, so people who use those should be already aware of pitfalls.


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vspinu commented Aug 2, 2016

Given that for a bulk of arithmetic operations with shorter periods is exact and that we currently use surpressWarnings to hide these warnings in a bunch of places, maintaining these warnings appears wrong to me.

So, unless you object I am removing them and adding methods for some arithmetic operations with periods which currently throw errors.


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hadley commented Aug 3, 2016

Yeah, I think it's a reasonable change.

@vspinu vspinu closed this in b95e21d Aug 17, 2016

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