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

The class np.ndindex is especially meant for this, and easily does the trick. Similar efficiency to the np.mesgrid method above, but it requires less code:

indices = np.array(list(np.ndindex(x.shape)))

For the dataframe, do:

df = pd.DataFrame({'V': x.flatten(), 'x': indices[:, 0], 'y': indices[:, 1]})

If you don't need the dataframe, just do list(np.ndindex(x.shape)).

Note: don't get confused between x (the array at hand), and 'x' (the name of the second column).

I know this question was posted a very long time ago, but just in case it's useful to anyone, as I didn't see np.ndindex being mentioned.