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rescale Date (and POSIXct) objects? #74

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zeehio opened this Issue Mar 1, 2016 · 3 comments

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@zeehio @hadley @karawoo
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zeehio commented Mar 1, 2016

Hi,

As of today it is not possible to rescale dates (and times) using scales::rescale. Is there a particular reason? I am puzzled by this, and I think that I must be missing something, because the problem seems fairly simple to tackle (using as.numeric somewhere). I would like to contribute with a clean solution, but I am afraid I am being naive on the solution. Any advice? (ggplot2::geom_segment could benefit from this)

days <- as.Date(c("2016-03-01", "2016-03-02", "2016-03-03"))
scales::rescale(days[2], from = c(days[1], days[3])) == 0.5

datetimes <- as.POSIXct(c("2016-03-01 08:05:00",
                          "2016-03-01 08:10:00",
                          "2016-03-01 08:15:00"),
                        format = "%Y-%m-%d %H:%M:%S")
testthat::expect_equal(scales::rescale(datetimes[2],
                       from = c(datetimes[1], datetimes[3])), 0.5)

I get

Error in Math.Date(x) : abs not defined for "Date" objects
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hadley commented Mar 1, 2016

I think coercing back and forth between numerics is probably the right thing to do.

@zeehio zeehio referenced this issue Mar 1, 2016

Merged

rescale() works with Date, POSIXct and POSIXlt objects. #75

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zeehio commented Nov 1, 2016

I just rebased the pull request on the latest master. Is there anything else I can do to get this merged?

Thank you for your time

@karawoo karawoo added the feature label Jun 14, 2017

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zeehio commented Jun 27, 2017

This is already merged in #75, so I will close it

@zeehio zeehio closed this Jun 27, 2017

zeehio added a commit to zeehio/scales that referenced this issue Oct 17, 2017

@zeehio
Provides a default method for rescale and rescale_mid 
The idea behind r-lib#74 and r-lib#75 was to use an S3 method so we never coerced to numeric
unless we had to. The aim was to prevent future cases similar to the integer64 one
where the coercion to numeric implies a loss of precision.

Reality is that there are many more numeric types out there that do not inherit from the numeric
class (like the dist object mentioned in r-lib#102), compared to the number of cases similar to integer64, where the numeric coercion is wrong.

So there is a trade-off:

- Not providing the `default` methods has the advantage that we only rescale the classes that we know can be rescaled, and the disadvantage that we get issues like r-lib#102 when we find a new object. This is what was implemented in r-lib#75

- Providing `rescale.default` and `rescale_mid.default` has the advantage of catching all the numeric-like types that do not inherit the numeric class (such as the dist object) and working as expected. However, we risk to deliver wrong results without an error if a case similar to integer64 appeared. This is what this commit implements.
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224db0e

@zeehio zeehio referenced this issue Oct 17, 2017

Merged

Provides a default method for rescale and rescale_mid #105

zeehio added a commit to zeehio/scales that referenced this issue Jun 9, 2018

@zeehio
Provides a default method for rescale and rescale_mid 
The idea behind r-lib#74 and r-lib#75 was to use an S3 method so we never coerced to numeric
unless we had to. The aim was to prevent future cases similar to the integer64 one
where the coercion to numeric implies a loss of precision.

Reality is that there are many more numeric types out there that do not inherit from the numeric
class (like the dist object mentioned in r-lib#102), compared to the number of cases similar to integer64, where the numeric coercion is wrong.

So there is a trade-off:

- Not providing the `default` methods has the advantage that we only rescale the classes that we know can be rescaled, and the disadvantage that we get issues like r-lib#102 when we find a new object. This is what was implemented in r-lib#75

- Providing `rescale.default` and `rescale_mid.default` has the advantage of catching all the numeric-like types that do not inherit the numeric class (such as the dist object) and working as expected. However, we risk to deliver wrong results without an error if a case similar to integer64 appeared. This is what this commit implements.
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