Change pandas group

Question

I have a DataFrame that I am grouping. I would like to add another column to the data frame, which is the result of the diff function for each group. Sort of:

df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar', 'foo', 'bar', 'foo', 'foo'], 'B' : ['one', 'one', 'two', 'three', 'two', 'two', 'one', 'three'], 'C' : np.random.randn(8), 'D' : np.random.randn(8)}) df_grouped = df.groupby('B') for name, group in df_grouped: new_df["D_diff"] = group["D"].diff() 

I would like to get the difference of column D for each group and have DF that includes a new column with diff calculation.

Answer 1

IIUC you can use DataFrameGroupBy.diff :

 df['D_diff'] = df.groupby('B')['D'].diff() print (df) ABCD D_diff 0 foo one 1.996084 0.580177 NaN 1 bar one 1.782665 0.042979 -0.537198 2 foo two -0.359840 1.952692 NaN 3 bar three -0.909853 0.119353 NaN 4 foo two -0.478386 -0.970906 -2.923598 5 bar two -1.289331 -1.245804 -0.274898 6 foo one -1.391884 -0.555056 -0.598035 7 foo three -1.270533 0.183360 0.064007