title: Pandas select the second to last column which is also not nan
body: <p>I've cleaned my data as much as I can and read them in Pandas dataframe. So the problem is that different files have different number of columns, but it always the second to the last non-nan column is what I want. So is there anyway to pick them out? Here is an example of the data.</p>
  
  <pre><code>     ...    f       g      h      l
  0    ...    39994  29.568  29.569 NaN  
  1    ...    39994  29.568  29.569 NaN  
  2    ...    39994  29.568  29.569 NaN 
  </code></pre>
  
  <p>so I want the column g in this case. So in other files, it could be f or anything depends on the number of nan columns in the end. But it's always the second to the last non-nan column is what I need. Thanks for the help ahead.</p>