title: can't understand scipy.sparse.csr_matrix example
body: <p>I can't wrap my head around <code>csr_matrix</code> examples in scipy documentation: <a href="https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csr_matrix.html" rel="noreferrer">https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csr_matrix.html</a></p>
  
  <p>Can someone explain how this example work?</p>
  
  <pre><code>&gt;&gt;&gt; row = np.array([0, 0, 1, 2, 2, 2])
  &gt;&gt;&gt; col = np.array([0, 2, 2, 0, 1, 2])
  &gt;&gt;&gt; data = np.array([1, 2, 3, 4, 5, 6])
  &gt;&gt;&gt; csr_matrix((data, (row, col)), shape=(3, 3)).toarray()
  array([[1, 0, 2],
         [0, 0, 3],
         [4, 5, 6]])
  </code></pre>
  
  <p>I believe this is following this format. </p>
  
  <p><code>csr_matrix((data, (row_ind, col_ind)), [shape=(M, N)])</code></p>
  
  <p><code>where data, row_ind and col_ind satisfy the relationship a[row_ind[k], col_ind[k]] = data[k].</code></p>
  
  <p>What is <code>a</code> here? </p>