Efficiently Creating A Pandas DataFrame From A Numpy 3d array

Here’s one approach that does most of the processing on NumPy before finally putting it out as a DataFrame, like so –

m,n,r = a.shape
out_arr = np.column_stack((np.repeat(np.arange(m),n),a.reshape(m*n,-1)))
out_df = pd.DataFrame(out_arr)

If you precisely know that the number of columns would be 2, such that we would have b and c as the last two columns and a as the first one, you can add column names like so –

out_df = pd.DataFrame(out_arr,columns=['a', 'b', 'c'])

Sample run –

>>> a
array([[[2, 0],
        [1, 7],
        [3, 8]],

       [[5, 0],
        [0, 7],
        [8, 0]],

       [[2, 5],
        [8, 2],
        [1, 2]],

       [[5, 3],
        [1, 6],
        [3, 2]]])
>>> out_df
    a  b  c
0   0  2  0
1   0  1  7
2   0  3  8
3   1  5  0
4   1  0  7
5   1  8  0
6   2  2  5
7   2  8  2
8   2  1  2
9   3  5  3
10  3  1  6
11  3  3  2