title: Pandas - adding columns, matching on index
body: <p>I'm trying to figure out if Panda's, when adding two series together, automatically matches on index or if it simply adds by element position.  If its just by position, is there a way to get it to add on index?  I've looked at merging, but I'm not very clear if the key in this case can be the index of both... </p>
  
  <p>For example, if I have do DFs:</p>
  
  <pre><code>df1 = index  value
          0      10
          1      12
          2      15
          4      20
  
  
  df2 = index  value
          0      10
          1      10
          3      10
          4      10
  </code></pre>
  
  <p>and I want to add <code>df1[total] = df1[value] + df2[value] =</code> </p>
  
  <pre><code>df1 = index  value
          0      20
          1      22
          2      15
          3      10
          4      30
  </code></pre>
  
  <p>Thanks for your help in advance!</p>