issues: 293345254

This data as json

id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
293345254 MDU6SXNzdWUyOTMzNDUyNTQ= 1875 roll doesn't handle periodic boundary conditions well 1217238 closed 0     5 2018-01-31T23:07:42Z 2018-08-15T08:11:29Z 2018-08-15T08:11:29Z MEMBER      

DataArray.roll() currently rolls both data variables and coordinates: ```

arr = xr.DataArray(range(4), [('x', range(0, 360, 90))]) arr.roll(x=2) <xarray.DataArray (x: 4)> array([2, 3, 0, 1]) Coordinates: * x (x) int64 180 270 0 90 ```

This is sort of makes sense, but the labels are now all non-monotonic, so you can't even plot the data with xarray. In my experience, you probably want coordinate labels that either look like:

  1. The unrolled original coordinates: [0, 90, 180, 270]
  2. Shifted coordinates: [-180, -90, 0, 90]

It should be easier to accomplish this is in xarray. I currently resort to using roll and manually fixing up coordinates after the fact.

I'm actually not sure if there are any use-cases for the current behavior. Choice (1) would have the virtue of being consistent with shift(): ```

arr.shift(x=2) <xarray.DataArray (x: 4)> array([nan, nan, 0., 1.]) Coordinates: * x (x) int64 0 90 180 270 ``` We could potentially add optional another argument for shifting labels, too, or requiring fixing that up by subtraction.

Note: you might argue that this is overly geoscience specific, and it would be, if this was only for handling a longitude coordinate. But periodic boundary conditions are common in many areas of the physical sciences.

 
{
    "url": "https://api.github.com/repos/pydata/xarray/issues/1875/reactions",
    "total_count": 1,
    "+1": 1,
    "-1": 0,
    "laugh": 0,
    "hooray": 0,
    "confused": 0,
    "heart": 0,
    "rocket": 0,
    "eyes": 0
}
   completed 13221727 issue

Links from other tables