Replace sub part of matrix by another small matrix in numpy

Question

I am new to Numpy and want to replace part of a matrix. For example, I have two matrices, A, B generated by numpy

In [333]: A = ones((5,5))

In [334]: A
Out[         


        

Answer 1

The following function replaces an arbitrary non-contiguous part of the matrix with another matrix.

def replace_submatrix(mat, ind1, ind2, mat_replace):
  for i, index in enumerate(ind1):
    mat[index, ind2] = mat_replace[i, :]
  return mat

Now an example of the application. We replace indices [1, 3] x [0, 3] (i.e. ind1 x ind2) of the empty 4 x 4 array x with the 2 x 2 array y of 4 different values:

x = np.full((4, 4), 0)
x
array([[0, 0, 0, 0],
       [0, 0, 0, 0],
       [0, 0, 0, 0],
       [0, 0, 0, 0]])

y = np.array([[1, 2], [5, 9]])
y
array([[1, 2],
       [5, 9]])

ind1 = [1, 3]
ind2 = [0, 3]
res = replace_submatrix(x, ind1, ind2, y)  
res  
array([[0, 0, 0, 0],
       [1, 0, 0, 2],
       [0, 0, 0, 0],
       [5, 0, 0, 9]])