How to perform grouped operations?

[joshua-gould]

I'm trying to compute grouped statistics from one data array and apply these statistics to another array.
Example:

import numpy as np
import xarray as xr

data = xr.DataArray(np.ones((10, 2)), dims=("y", "x"), name="", coords={'y': [1] * 5 + [2] * 5})

mean = data.groupby('y').mean()
data2 = xr.DataArray(np.ones((6, 2)), dims=("y", "x"), name="", coords={'y': [2] * 3 + [1] * 3})
# trying subtract group means from data to matching groups in data2
# ValueError: cannot reindex or align along dimension 'y' because the (pandas) index has duplicate values
data2 = data2 - mean

[dcherian]

Does https://docs.xarray.dev/en/stable/user-guide/groupby.html#grouped-arithmetic help?

It should be data2.groupby("y") - mean

[joshua-gould]

Thanks so much. I read the documentation before but I missed the example code ds.groupby("letters") - alt

[joshua-gould]

How do I perform 2 operations (e.g. substract mean and divide by standard deviation)? Do I need to group again?

import numpy as np
import xarray as xr

data = xr.DataArray(np.random.random((10, 2)), dims=("obs", "var"), name="", coords={'obs': [1] * 5 + [2] * 5})
grouped = data.groupby('obs')
mean = grouped.mean()
std = grouped.std()
# fails
# data = (grouped - mean) / std

# need to group again?
data = grouped - mean
data = data.groupby("obs") - std