Flatten numpy array but also keep index of value positions?

Question

I have several 2D numpy arrays (matrix) and for each one I would like to convert it to vector containing the values of the array and a vector containing each row/column index.

Answer 1

You could also let pandas do the work for you since you'll be using it in a dataframe:

x = np.array([[3, 1, 4],
              [1, 5, 9],
              [2, 6, 5]])
df=pd.DataFrame(x)
#unstack the y columns so that they become an index then reset the
#index so that indexes become columns.
df=df.unstack().reset_index()
df

   level_0  level_1  0
0        0        0  3
1        0        1  1
2        0        2  2
3        1        0  1
4        1        1  5
5        1        2  6
6        2        0  4
7        2        1  9
8        2        2  5

#name the columns and switch the column order
df.columns=['x','y','V']
cols = df.columns.tolist()
cols = cols[-1:] + cols[:-1]
df = df[cols]
df

   V  x  y
0  3  0  0
1  1  0  1
2  2  0  2
3  1  1  0
4  5  1  1
5  6  1  2
6  4  2  0
7  9  2  1
8  5  2  2