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.

👍

Looks like the idea of a glossary is already being discussed in https://github.com/pydata/xarray/issues/2410.

Thanks for these answers! On a related point, I'd be keen to open a PR for improved documentation for whatever object a is. It seems like the documented Xarray terminology is "multidimensional coordinate", right? To me, "non-index coordinate" and "multidimensional coordinate" are both pretty vague until you're more familiar with Xarray's way of thinking.

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. 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?

💯

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"?

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 explanation. I'll create a PR and link to this issue this evening.

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.