title: Dataframe: cannot replace elements with others in DF
body: <p>I have imported data with read_csv and have a DataFrame called <strong>praw</strong>. It has several columns, like 'TIME', 'COUNTRY', 'PRODUCT' and 'VALUE'.</p>
  
  <p>I want to replace the missing data for 'PRODUCT' == product1 with the data I have for product2, given a selected year and country.</p>
  
  <p>I use the following code</p>
  
  <pre><code>praw.loc[(praw['COUNTRY'] == country) &amp; (praw['TIME'] == year) &amp; 
  (praw['PRODUCT'] == 'product1'),'Value'] = praw.loc[(praw['COUNTRY'] == country) &amp; 
  (praw['TIME'] == year) &amp; (praw['PRODUCT'] == 'product2'),'Value']
  </code></pre>
  
  <p>If I print </p>
  
  <pre><code>praw.loc[(praw['COUNTRY'] == country) &amp; 
  (praw['TIME'] == year) &amp; 
  (praw['PRODUCT'] == 'product1'),'Value'] 
  </code></pre>
  
  <p>I still have NaN. What am I doing wrong?</p>
  
  <p>I am sure the value for 'product2' is not missing :)</p>