Question

I was wondering if it was possible to create a nested matrix in python. Here I define my matrix A

  A = array([[ 12.,   0.],[  0.,   4.]])

I would like to replace the zeros with a generic 2×2 matrix and then plot everything with imshow(). Is that possible?

I tried defining my nested matrix this way

  A = array([[ 12.,   array([[ 1.,   1.],[  1.,   1.]])],[  0.,   4.]])

but I got this error message

ValueError: setting an array element with a sequence.

Answer 1

>>> M = NP.empty((5, 5), dtype=NP.object)     # a 2D NumPy array
>>> M
  array([[None, None, None, None, None],
         [None, None, None, None, None],
         [None, None, None, None, None],
         [None, None, None, None, None],
         [None, None, None, None, None]], dtype=object)

Now you can insert sequences, without getting the ValueError

>>> M[2,2] = NP.array([4, 3, 5])
>>> M
  array([[None, None, None, None, None],
         [None, None, None, None, None],
         [None, None, [4 3 5], None, None],
         [None, None, None, None, None],
         [None, None, None, None, None]], dtype=object)

>>> M[2,2]
   array([4, 3, 5])

The other part of the OP–passing an Array like this to Matplotlib’s imshow, is a problem.

imshow visually represents a 2D array as a cloud of points positioned on the canvas according to their x, y indices. The value at that index is indicated according to different colors and color intensities based on a colormap which maps color to array value. Therefore, valid arguments for imshow’s data parameter are:

• NumPy 2D arrays

NumPy arrays of higher dimension in two (and only these two AFAIK) which imshow can interpret as

• NumPy 2D array of rgb tuples (x, y, r, b, g)

• NumPy 6D arrays, which are interpreted as a 2D array of rgba
  tuples (x, y, r, g, b, a)