How to get maximum coordinates in xarray?

Question

simple question: I not only want the maximum value, but also its coordinates in xarray DataArray. How to do it?

I can, of course, write my own simple funtion effect, but I wonder if there is anything built into xarray?

Answer 1

You can use da.where()to filter by maximum value:

In [15]: da = xr.DataArray(np.random.rand(2,3,4))

In [16]: da.where(da==da.max(), drop=True).squeeze()
Out[16]: 
<xarray.DataArray (dim_0: 1, dim_1: 1, dim_2: 1)>
array([[[ 0.91077406]]])
Coordinates:
  * dim_0    (dim_0) int64 0
  * dim_1    (dim_1) int64 2
  * dim_2    (dim_2) int64 3

Answer 2

idxmax()would be very desirable in xarray, but so far no one bothered to implement it.

At the moment, you can find the coordinates of the maximum by combining argmaxand isel:

>>> array = xarray.DataArray(
...    [[1, 2, 3], [3, 2, 1]],
...    dims=['x', 'y'],
...    coords={'x': [1, 2], 'y': ['a', 'b', 'c']})

>>> array
<xarray.DataArray (x: 2, y: 3)>
array([[1, 2, 3],
       [3, 2, 1]])
Coordinates:
  * x        (x) int64 1 2
  * y        (y) <U1 'a' 'b' 'c'

>>> array.isel(y=array.argmax('y'))
<xarray.DataArray (x: 2)>
array([3, 3])
Coordinates:
  * x        (x) int64 1 2
    y        (x) <U1 'c' 'a'

, , , .max() ! , .

, , :

>>> array.argmax()  # what??
<xarray.DataArray ()>
array(2)

, , np.argmax. , , - , . - . .

Answer 3

:

, , , , .

stackdata = data.stack(z=('lon', 'lat'))
maxi = stackdata.argmax(axis=1)
maxipos = stackdata['z'][maxi]
lonmax = [maxipos.values[itr][0] for itr in range(ntime)]
latmax = [maxipos.values[itr][1] for itr in range(ntime)]