title: In pandas how to calculate 'Countif' on a moving window basis?
body: <p>Given</p>
  
  <pre><code>A = pd.DataFrame([[1, 5, -2], [2, 4, -4], [3, 3, -1], [4, 2, 2], [5, 1, 4]],
               columns=['A', 'B', 'C'], index=[1, 2, 3, 4, 5])
  </code></pre>
  
  <p>Say you want to calculate the number observations &lt; 0 in the column C over the last 3 observations on a rolling basis. In excel you would slide a 'countif' calculation over a specified window with the condition and the desired result would be:</p>
  
  <pre><code>D = # of x &lt; 0 on a rolling window basis of size 3
  
  A
  Out[79]: 
     A  B  C  D
  1  1  5 -2  
  2  2  4 -4
  3  3  3 -1  3
  4  4  2  2  2
  5  5  1  4  1
  </code></pre>
  
  <p>How can I do this in an efficient(Pythonic) way using Pandas?</p>
  
  <p>Thanks</p>