title: adding value to a subset of a column in Pandas
body: <p>For the following data frame:</p>
  
  <pre><code>index    A    B       C   
  0        3    word    7
  1        4    type    3
  2        8    manic   4
  3        9    tour    6
  </code></pre>
  
  <p>I am going to add value to a subset of column A. This is my code:</p>
  
  <pre><code>df.A=  df.loc[0:2, 'A' ] + 30
  </code></pre>
  
  <p>But this is there result:</p>
  
  <pre><code>  index       A      B       C   
      0        33    word    7
      1        34    type    3
      2        38    manic   4
      3              tour    6
  </code></pre>
  
  <p>makes the value of the fourth row of column A "null". Any suggestion ? </p>