Question

I have a matrix A and I want 2 matrices U and L such that U contains the upper triangular elements of A (all elements above and not including diagonal) and similarly for L(all elements below and not including diagonal). Is there a numpy method to do this?

e.g

A = array([[ 4.,  9., -3.],
           [ 2.,  4., -2.],
           [-2., -3.,  7.]])

U = array([[ 0.,  9., -3.],
           [ 0.,  0., -2.],
           [ 0.,  0.,  0.]])

L = array([[ 0.,  0.,  0.],
           [ 2.,  0.,  0.],
           [-2., -3.,  0.]])

Answer 1

Try numpy.triu (triangle-upper) and numpy.tril (triangle-lower).

Code example:

np.triu([[1,2,3],[4,5,6],[7,8,9],[10,11,12]])
array([[ 1,  2,  3],
       [ 4,  5,  6],
       [ 0,  8,  9],
       [ 0,  0, 12]])