Create a 2D array from another array and its indices with NumPy

Question

Given an array:

arr = np.array([[1, 3, 7], [4, 9, 8]]); arr

array([[1, 3, 7],
       [4, 9, 8]])

And given its indices:

np         


        

Answer 1

A more generic answer for nd arrays, that handles other dtypes correctly:

def indices_merged_arr(arr):
    out = np.empty(arr.shape, dtype=[
        ('index', np.intp, arr.ndim),
        ('value', arr.dtype)
    ])
    out['value'] = arr
    for i, l in enumerate(arr.shape):
        shape = (1,)*i + (-1,) + (1,)*(arr.ndim-1-i)
        out['index'][..., i] = np.arange(l).reshape(shape)
    return out.ravel()

This returns a structured array with an index column and a value column, which can be of different types.