title: Replacing row values in pandas
body: <p>I would like to replace row values in pandas. </p>
  
  <p>In example:</p>
  
  <pre><code>import pandas as pd
  import numpy as np    
  
  a = np.array(([100, 100, 101, 101, 102, 102],
                   np.arange(6)))
  pd.DataFrame(a.T)
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>array([[100,   0],
         [100,   1],
         [101,   2],
         [101,   3],
         [102,   4],
         [102,   5]])
  </code></pre>
  
  <p>Here, I would like to replace the rows with the values <code>[101, 3]</code> with <code>[200, 10]</code> and the result should therefore be:</p>
  
  <pre><code>array([[100,   0],
         [100,   1],
         [101,   2],
         [200,  10],
         [102,   4],
         [102,   5]])
  </code></pre>
  
  <p><strong>Update</strong></p>
  
  <p>In a more general case I would like to replace multiple rows.</p>
  
  <p>Therefore the old and new row values are represented by nx2 sized matrices (n is number of row values to replace). In example:</p>
  
  <pre><code>old_vals = np.array(([[101, 3]],
                       [[100, 0]],
                       [[102, 5]]))
  
  new_vals = np.array(([[200, 10]],
                       [[300, 20]],
                       [[400, 30]]))
  </code></pre>
  
  <p>And the result is:</p>
  
  <pre><code>array([[300,  20],
         [100,   1],
         [101,   2],
         [200,  10],
         [102,   4],
         [400,  30]])
  </code></pre>