issues: 484622545

This works with isel and sel (see below). There's no particular reason why it can't work with reindex and interp, too, though we would need to implement our own vectorized version of linear interpolation (should not be too bad, it's mostly a matter of indexing twice and calculating weights from the difference in coordinate values).

Apparently this is quite important for vertical regridding in weather/climate science (cc @rabernat, @nbren12 ) ``` In [35]: import xarray as xr

In [36]: import numpy as np

In [37]: data = xr.DataArray(np.arange(12).reshape((3, 4)), [('x', np.arange(3)), ('y', np.arange(4))])

In [38]: ind = xr.DataArray([[0, 2], [1, 0], [1, 2]], dims=['x', 'z'], coords={'x': [0, 1, 2]})

In [39]: data Out[39]: <xarray.DataArray (x: 3, y: 4)> array([[ 0, 1, 2, 3], [ 4, 5, 6, 7], [ 8, 9, 10, 11]]) Coordinates: * x (x) int64 0 1 2 * y (y) int64 0 1 2 3

In [40]: ind Out[40]: <xarray.DataArray (x: 3, z: 2)> array([[0, 2], [1, 0], [1, 2]]) Coordinates: * x (x) int64 0 1 2 Dimensions without coordinates: z

In [41]: data.isel(y=ind) Out[41]: <xarray.DataArray (x: 3, z: 2)> array([[ 0, 2], [ 5, 4], [ 9, 10]]) Coordinates: * x (x) int64 0 1 2 y (x, z) int64 0 2 1 0 1 2 Dimensions without coordinates: z

In [42]: data.sel(y=ind) Out[42]: <xarray.DataArray (x: 3, z: 2)> array([[ 0, 2], [ 5, 4], [ 9, 10]]) Coordinates: * x (x) int64 0 1 2 y (x, z) int64 0 2 1 0 1 2 Dimensions without coordinates: z

In [43]: data.interp(y=ind)

ValueError Traceback (most recent call last) <ipython-input-43-e6eb7e39fd31> in <module> ----> 1 data.interp(y=ind)

~/dev/xarray/xarray/core/dataarray.py in interp(self, coords, method, assume_sorted, kwargs, coords_kwargs) 1303 kwargs=kwargs, 1304 assume_sorted=assume_sorted, -> 1305 coords_kwargs 1306 ) 1307 return self._from_temp_dataset(ds)

~/dev/xarray/xarray/core/dataset.py in interp(self, coords, method, assume_sorted, kwargs, coords_kwargs) 2455 } 2456 variables[name] = missing.interp( -> 2457 var, var_indexers, method, kwargs 2458 ) 2459 elif all(d not in indexers for d in var.dims):

~/dev/xarray/xarray/core/missing.py in interp(var, indexes_coords, method, *kwargs) 533 else: 534 out_dims.add(d) --> 535 return result.transpose(tuple(out_dims)) 536 537

~/dev/xarray/xarray/core/variable.py in transpose(self, *dims) 1219 return self.copy(deep=False) 1220 -> 1221 data = as_indexable(self._data).transpose(axes) 1222 return type(self)(dims, data, self._attrs, self._encoding, fastpath=True) 1223

~/dev/xarray/xarray/core/indexing.py in transpose(self, order) 1218 1219 def transpose(self, order): -> 1220 return self.array.transpose(order) 1221 1222 def getitem(self, key):

ValueError: axes don't match array

In [44]: data.reindex(y=ind) /Users/shoyer/dev/xarray/xarray/core/dataarray.py:1240: FutureWarning: Indexer has dimensions ('x', 'z') that are different from that to be indexed along y. This will behave differently in the future. fill_value=fill_value,

---

ValueError Traceback (most recent call last) <ipython-input-44-1277ead996ae> in <module> ----> 1 data.reindex(y=ind)

~/dev/xarray/xarray/core/dataarray.py in reindex(self, indexers, method, tolerance, copy, fill_value, **indexers_kwargs) 1238 tolerance=tolerance, 1239 copy=copy, -> 1240 fill_value=fill_value, 1241 ) 1242 return self._from_temp_dataset(ds)

~/dev/xarray/xarray/core/dataset.py in reindex(self, indexers, method, tolerance, copy, fill_value, **indexers_kwargs) 2360 tolerance, 2361 copy=copy, -> 2362 fill_value=fill_value, 2363 ) 2364 coord_names = set(self._coord_names)

~/dev/xarray/xarray/core/alignment.py in reindex_variables(variables, sizes, indexes, indexers, method, tolerance, copy, fill_value) 398 ) 399 --> 400 target = new_indexes[dim] = utils.safe_cast_to_index(indexers[dim]) 401 402 if dim in indexes:

~/dev/xarray/xarray/core/utils.py in safe_cast_to_index(array) 104 index = array 105 elif hasattr(array, "to_index"): --> 106 index = array.to_index() 107 else: 108 kwargs = {}

~/dev/xarray/xarray/core/dataarray.py in to_index(self) 545 arrays. 546 """ --> 547 return self.variable.to_index() 548 549 @property

~/dev/xarray/xarray/core/variable.py in to_index(self) 445 def to_index(self): 446 """Convert this variable to a pandas.Index""" --> 447 return self.to_index_variable().to_index() 448 449 def to_dict(self, data=True):

~/dev/xarray/xarray/core/variable.py in to_index_variable(self) 438 """Return this variable as an xarray.IndexVariable""" 439 return IndexVariable( --> 440 self.dims, self._data, self._attrs, encoding=self._encoding, fastpath=True 441 ) 442

~/dev/xarray/xarray/core/variable.py in init(self, dims, data, attrs, encoding, fastpath) 1941 super().init(dims, data, attrs, encoding, fastpath) 1942 if self.ndim != 1: -> 1943 raise ValueError("%s objects must be 1-dimensional" % type(self).name) 1944 1945 # Unlike in Variable, always eagerly load values into memory

ValueError: IndexVariable objects must be 1-dimensional ```