python - Groupby an numpy.array based on groupby of a pandas.DataFrame with the same length -

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i have numpy.array arr , pandas.dataframe df.

arr , df have same shape (x,y).

i need group 1 column of df , apply transformation of impacted rows on arr have same shape.

to clear, here toy example:

arr =     0  1   12   3    2  5   45   47    3  19  11  111  df =     b   c   d 0  0  1   2   3 1  4  5   6   7 2  4  9  10  11

i want group df a , compute mean in place of transforming df want arr transformed.

so like:

    arr =          0        1         12          3        (2+3)/2  (5+19)/2   (45+11)/2   (47+111)/2

is possible? no expensive loops?

thanks in advance

it looks need first create dataframe arr, groupby column a , aggregate mean. last convert numpy array values:

print (pd.dataframe(arr).groupby(df.a).mean().values) [[  0.    1.   12.    3. ]  [  2.5  12.   28.   79. ]]

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