title: make a new variable from existing data in pandas dataframe
body: <p>I need some help with forming a new variable in a dataframe
  I have a very large dataset were i want to make a new variable from existing data in another column.
  Here is an example.</p>
  
  <pre><code>&gt;&gt;&gt; df
     A  B   
  0  a  1  
  1  b  2  
  2  c  3  
  3  a  5  
  4  d  6  
  </code></pre>
  
  <p>I wwant to create a new column with the values a and c in A and name them ac, and the b and d and name them bd
  Final result should look like. Kind of grouping them from A and give the group a new name in a new column. </p>
  
  <pre><code>&gt;&gt;&gt; df
     A  B   C
  0  a  1  ac
  1  b  2  bd
  2  c  3  ac
  3  a  5  ac
  4  d  6  bd
  </code></pre>
  
  <p>/Jonas</p>