'Initializes the data structures for the cached matrix.
Parameters
constraints : list
A list of constraints in the matrix.
x_length : int
The number of columns in the matrix.
Returns
((V, I, J), array)'

def _init_matrix_cache(self, constraints, x_length):

    rows = sum([(c.size[0] * c.size[1]) for c in constraints])
    COO = ([], [], [])
    const_vec = self.vec_intf.zeros(rows, 1)
    return MatrixCache(COO, const_vec, constraints, x_length)
