issue_comments

Good work on finding that issue. I think even if we can get something brief in, that would be helpful.

On the specific definitions:

What do you think of the terminology "alternative" or "auxiliary" dimension? a is clearly a dimension in the sense that it has coordinates or labels for all the "tick marks" along the x dimension.

For me 'dimension' has a precise definition from traditional sciences, so having our 'coordinate' be an additional / auxiliary / alternative dimension wouldn't be consistent with that (e.g. a 4-dimensional array would still be 4 dimensional regardless of how many coordinates it had).

At the very least, I'd love to add a lot more examples of how to actually use these things.

👍

Looking at this now, and I'm a little surprised at the verbiage. In your example, do you consider a to be a "variable"? I thought variables were individual DataArray objects "inside" Dataset objects. My colleagues and I have been referring to objects such as a as "alternative" or "auxiliary" dimensions. Basically, a different labeling of the same coordinates. You also seem to call these "multidimensional coordinates"?

You're not alone; the proliferation and overlap of terms can be confusing at the least. Maybe we should have a glossary somewhere. Briefly: - Dimensions are like x & y above (a is not a dimension) - Coordinates are labels along dimensions. These can be either index or non-index coordinates. a above is an non-index coordinate; x & y are indexes. Currently indexes are always named the same as their dimension. - (not 100% sure about this one, @pydata/xarray correct me where I'm wrong) Data Variables are indeed the objects inside a dataset. All the objects are Variables, including Coordinates.

But I do think I see the use case. The point is that you can take an existing dimension's coordinates and set them as the coordinates for an alternative dimension?

💯

Thanks for the issue @gwgundersen

I think the docs are potentially a bit unclear, and maybe the error message. The existing intention of set_index is to set existing variables as indexes, rather than creating new ones. For example, to extend your case:

```python

In [16]: arr = xr.DataArray(data=np.ones((2, 3)), dims=['x', 'y'], coords={'x': range(2), 'y': range(3), 'a': ('x', [3,4])})

In [17]: arr Out[17]: <xarray.DataArray (x: 2, y: 3)> array([[1., 1., 1.], [1., 1., 1.]]) Coordinates: * x (x) int64 0 1 * y (y) int64 0 1 2 a (x) int64 3 4

In [18]: arr.set_index(x='a') Out[18]: <xarray.DataArray (x: 2, y: 3)> array([[1., 1., 1.], [1., 1., 1.]]) Coordinates: * x (x) int64 3 4 * y (y) int64 0 1 2 ```

We'd definitely be keen on a PR improving the error message (i.e. something like 'a' is not the name of an existing variable), and v open to feedback on the docs & the method's functionality; let us know if you'd be interested in that PR.