title: Pandas: Re-assigning values to a 'block' of a Data frame
body: <p>Probably a trivial problem, but I need to understand what's going on here (and how to fix it).</p>
  
  <p>Suppose I have a dataframe with columns 'a' and 'b', as follows:</p>
  
  <pre><code>f = pandas.DataFrame({'a':[1,2,3,4,5], 'b':[10,20,30,40,50]})
  </code></pre>
  
  <p>Now for every element of 'a' that is 3 or less, I want to divide the corresponding elements of 'b' by 10. </p>
  
  <pre><code>f[f['a']&lt;=3]['b'] = (f[f['a']&lt;=3]['b'])/10
  </code></pre>
  
  <p>So that the values in column 'b' should now be [1,2,3,40,50].</p>
  
  <p>But I find that column 'b' remains unchanged! What gives?</p>