How to get the coordinates of the maximum in xarray?

Question

Simple question: I don't only want the value of the maximum but also the coordinates of it in an xarray DataArray. How to do that?

I can, of course, write my own simple reduce function, but I wonder if there is anything built-in in xarray?

Answer 1

Update:

xarray now has the idxmax method for selecting the coords of the max values along one dimension:

In [8]: da = xr.DataArray(
   ...:     np.random.rand(2,3),
   ...:     dims=list('ab'),
   ...:     coords=dict(a=list('xy'), b=list('ijk'))
   ...: )


In [14]: da
Out[14]:
<xarray.DataArray (a: 2, b: 3)>
array([[0.63059257, 0.00155463, 0.60763418],
       [0.19680788, 0.43953352, 0.05602777]])
Coordinates:
  * a        (a) <U1 'x' 'y'
  * b        (b) <U1 'i' 'j' 'k'

In [13]: da.idxmax('a')
Out[13]:
<xarray.DataArray 'a' (b: 3)>
array(['x', 'y', 'x'], dtype=object)
Coordinates:
  * b        (b) <U1 'i' 'j' 'k'

The below answer is still relevant for the maximum over multiple dimensions, though.

---

You can use da.where() to filter based on the max value:

In [17]: da = xr.DataArray(
             np.random.rand(2,3), 
             dims=list('ab'), 
             coords=dict(a=list('xy'), b=list('ijk'))
         )

In [18]: da.where(da==da.max(), drop=True).squeeze()
Out[18]:
<xarray.DataArray ()>
array(0.96213673)
Coordinates:
    a        <U1 'x'
    b        <U1 'j'

Edit: updated the example to show the indexes more clearly, now that xarray doesn't have default indexes