Delete diagonals of zero elements

Question

I'm trying to reshape an array from its original shape, to make the elements of each row descend along a diagonal:

np.random.seed(0) 
my_array = np.random.randint(1, 50, size=(5, 3))

array([[45, 48,  1],
       [ 4,  4, 40],
       [10, 20, 22],
       [37, 24,  7],
       [25, 25, 13]])

I would like the result to look like this:

my_array_2 = np.array([[45,  0,  0],
                       [ 4, 48,  0],
                       [10,  4,  1],
                       [37, 20, 40],
                       [25, 24, 22],
                       [ 0, 25,  7],
                       [ 0,  0, 13]])

This is the closest solution I've been able to get:

my_diag = []
for i in range(len(my_array)):
    my_diag_ = np.diag(my_array[i], k=0)
    my_diag.append(my_diag_)
my_array1 = np.vstack(my_diag)

array([[45,  0,  0],
       [ 0, 48,  0],
       [ 0,  0,  1],
       [ 4,  0,  0],
       [ 0,  4,  0],
       [ 0,  0, 40],
       [10,  0,  0],
       [ 0, 20,  0],
       [ 0,  0, 22],
       [37,  0,  0],
       [ 0, 24,  0],
       [ 0,  0,  7],
       [25,  0,  0],
       [ 0, 25,  0],
       [ 0,  0, 13]])

From here I think it might be possible to remove all zero diagonals, but I'm not sure how to do that.

Answer 1

One way using numpy.pad:

n = my_array.shape[1] - 1
np.dstack([np.pad(a, (i, n-i), "constant") 
           for i, a in enumerate(my_array.T)])

Output:

array([[[45,  0,  0],
        [ 4, 48,  0],
        [10,  4,  1],
        [37, 20, 40],
        [25, 24, 22],
        [ 0, 25,  7],
        [ 0,  0, 13]]])

Answer 2

In [134]: arr = np.array([[45, 48,  1],
     ...:        [ 4,  4, 40],
     ...:        [10, 20, 22],
     ...:        [37, 24,  7],
     ...:        [25, 25, 13]])
In [135]: res= np.zeros((arr.shape[0]+arr.shape[1]-1, arr.shape[1]), arr.dtype)

Taking a hint from how np.diag indexes a diagonal, iterate on the rows of arr:

In [136]: for i in range(arr.shape[0]):
     ...:     n = i*arr.shape[1]
     ...:     m = arr.shape[1]
     ...:     res.flat[n:n+m**2:m+1] = arr[i,:]
     ...: 
In [137]: res
Out[137]: 
array([[45,  0,  0],
       [ 4, 48,  0],
       [10,  4,  1],
       [37, 20, 40],
       [25, 24, 22],
       [ 0, 25,  7],
       [ 0,  0, 13]])