title: Need count of negative values in a dataframe
body: <p>I need total count of negative values in a dataframe. i am able to get for an array but unable to find for DataFrame. for array i am using below code can any one suggest me how to get the count for below DataFrame.</p>
  
  <pre><code>sum(n&lt;0 for n in numbers)
  </code></pre>
  
  <p>Below is my dataframe and expected result is 4</p>
  
  <pre><code>  a  b  c  d
     -3 -2 -1  1
     -2  2  3  4
      4  5  7  8
  </code></pre>