title: How to get the first index of a pandas DataFrame for which several undefined columns are not null?
body: <p>I have a dataframe with several columns. I want to get the first row index for which:</p>
  
  <ul>
  <li>the value of column A is not null</li>
  <li>there exists at least n other columns for which the values are not null</li>
  </ul>
  
  <p>example: if my dataframe is:</p>
  
  <pre><code>          Date          A             B      C     D
  0   2015-01-02          NaN           1      1    NaN
  1   2015-01-02          NaN           2      2    NaN
  2   2015-01-02          NaN           3      3    NaN
  3   2015-01-02          1            NaN     4    NaN
  5   2015-01-02          NaN           2      NaN  NaN
  6   2015-01-03          1            NaN     6    NaN
  7   2015-01-03          1             1      6    NaN
  8   2015-01-03          1             1      6     8
  </code></pre>
  
  <p>If n=1 I would get 3</p>
  
  <p>If n=2 I would get 7</p>
  
  <p>If n=3 I would get 8</p>