title: Conditional indexing with Numpy ndarray
body: <p>I have a Numpy ndarray matrix of float values and I need to select spesific rows where certain columns have values satisfying certain criteria. For example lets say I have the following numpy matrix: </p>
  
  <pre><code>matrix = np.ndarray([4, 5])
  matrix[0,:] = range(1,6)
  matrix[1,:] = range(6,11)
  matrix[2,:] = range(11,16)
  matrix[3,:] = range(16,21)
  </code></pre>
  
  <p>Lets say I want to select rows from the matrix where the first column's value is between 1 and 6 and the value of second column is between 2-7. </p>
  
  <p>How can I get the row-indexes of the matrix where these conditions are satisfied? What about if I want to delete the rows that satisfy the conditional criterion? </p>