title: Updating a coo matrix on the fly with scipy
body: <p>I have not found a solution to this problem after searching the site. Its quite simple, I would like to update an already existing coo sparse matrix. So lets say I have initiated a coo matrix:</p>
  
  <pre><code>from scipy.sparse import coo_matrix
  import numpy as np
  row  = np.array([0, 3, 1, 0])
  col  = np.array([0, 3, 1, 2])
  data = np.array([4, 5, 7, 9])
  a=coo_matrix((data, (row, col)), shape=(4, 4)).toarray()
  array([[4, 0, 9, 0],
        [0, 7, 0, 0],
        [0, 0, 0, 0],
        [0, 0, 0, 5]])
  </code></pre>
  
  <p>Fine but what if I just want an empty sparse array and initiate it with only the shape, and then update the values many times. The only way I have succeeded is to add a new coo matrix to my old one</p>
  
  <pre><code>a=coo_matrix((4, 4), dtype=np.int8)
  a=a+coo_matrix((data, (row, col)), shape=(4, 4))
  a.toarray()
  array([[4, 0, 9, 0],
        [0, 7, 0, 0],
        [0, 0, 0, 0],
        [0, 0, 0, 5]])
  </code></pre>
  
  <p>And I would like to update this sparse array many times. But this takes quite awhile since I am calling upon the coo function for each update. There has to be a better way but I feel like the documentation is a little light (at least what I have read) or that I am just not seeing it. </p>
  
  <p>Thanks very much</p>