title: Multiplying specific columns of dataframe by constant scalar value
body: <p>How do I multiply only specific columns of a dataframe by a constant value?</p>
  
  <pre><code>df0 = pd.DataFrame({'A' : 1.,
                      'B' : 1,
                      'C' : 1,
                      'D' : np.array([1] * 4,dtype='int32')})
  
  mult_by_two = df0.iloc[:,2:].mul(2)
  print mult_by_two
  </code></pre>
  
  <p>I get this:</p>
  
  <pre><code>   C  D
  0  2  2
  1  2  2
  2  2  2
  3  2  2
  </code></pre>
  
  <p>but what I want is this:</p>
  
  <pre><code>   A  B  C  D
  0  1  1  2  2
  1  1  1  2  2
  2  1  1  2  2
  3  1  1  2  2
  </code></pre>