title: Pandas Data-Frame : Conditionally select columns
body: <p>I have a pandas data-frame as shown below...</p>
  
  <pre><code>   C1  C2   C3     C4
  0  -1   -3   3   0.75
  1  10   20  30  -0.50 
  </code></pre>
  
  <p>I want to add only the first two columns in each row which have a value less than zero. For example, the series I would get for the above case would be as below...</p>
  
  <pre><code>   CR
  0  -4
  1  0
  </code></pre>
  
  <p>I know how to apply other functions like the below...</p>
  
  <pre><code>df.iloc[:, :-2].abs().sum(axis = 1)
  </code></pre>
  
  <p>Is there a way using lambda functions?</p>