title: Change an element in the last row of a DataFrame
body: <p>I set up a simple DataFrame in pandas:</p>
  
  <pre><code>a = pandas.DataFrame([[1,2,3], [4,5,6], [7,8,9]], columns=['a','b','c'])
  &gt;&gt;&gt; print a
     a  b  c
  0  1  2  3
  1  4  5  6
  2  7  8  9
  </code></pre>
  
  <p>I would like to be able to alter a single element in the last row of. In pandas==0.13.1 I could use the following:</p>
  
  <pre><code>a.iloc[-1]['a'] = 77
  &gt;&gt;&gt; print a
      a  b  c
  0   1  2  3
  1   4  5  6
  2  77  8  9
  </code></pre>
  
  <p>but after updating to pandas==0.14.1, I get the following warning when doing this: </p>
  
  <pre><code>SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
  Try using .loc[row_index,col_indexer] = value instead
  </code></pre>
  
  <p>The problem of course being that -1 is not an index of <code>a</code>, so I can't use <code>loc</code>. As the warning indicates, I have not changed column <code>'a'</code> of the last row, I've only altered a discarded local copy. </p>
  
  <p>How do I do this in the newer version of pandas? I realize I could use the index of the last row like:</p>
  
  <pre><code>a.loc[2,'a'] = 77
  </code></pre>
  
  <p>But I'll be working with tables where multiple rows have the same index, and I don't want to reindex my table every time. Is there a way to do this without knowing the index of the last row before hand?</p>