title: Backward Concatenation of Column values(Strings) in Python pandas
body: <p>I trying to concatenate strings by decremented columns and assuming the table is as below. I'm creating fixed number (5) of NEW columns &amp; want every column to have string concatenated by non-empty strings. Let say for the <code>row-1</code>, first concatenation will be <code>STR_1;STR_2;STR_3</code> and the second concatenation should be <code>STR_1;STR_2</code> and third concatenation should be <code>STR_1</code> &amp; the rest of the cells should remain empty.</p>
  
  <pre><code>MAJOR_BRAND | SUB_SEGMENT | PACKAGE_TYPE | PACKAGE |  SIZE   | FLAVOR  | VOLUME
  -------------------------------------------------------------------------------------
  STR_1       | STR_2       | STR_3        |         |         |         |
  STR_1       | STR_2       | STR_3        |         |         |         | STR_7
  STR_1       |             |              | STR_4   | STR_5   | STR_6   | STR_7
  STR_1       |             | STR_3        |         |         | STR_6   | STR_7
  STR_1       |             | STR_3        |         |         | STR_6   |    
  </code></pre>
  
  <p>Output should looks like</p>
  
  <pre><code> STR_JOIN_1                   | STR_JOIN_2             | STR_JOIN_3      | STR_JOIN_4 | STR_JOIN_5
  ----------------------------------------------------------------------------------------------------
  STR_1;STR_2;STR_3             |STR_1;STR_2             |STR_1            |            |
  STR_1;STR_2;STR_3;STR_7       |STR_1;STR_2;STR_3       |STR_1;STR_2      |STR_1       |       
  STR_1;STR_4;STR_5;STR_6;STR_7 |STR_1;STR_4;STR_5;STR_6 |STR_1;STR_4;STR_5|STR_1;STR_4 | STR_1   
  STR_1;STR_3;STR_6;STR_7       |STR_1;STR_3;STR_6       |STR_1;STR_3      |STR_1       |
  STR_1;STR_3;STR_6             |STR_1;STR_3             |STR_1            |            | 
  </code></pre>
  
  <p>Kind help will be greatly appreciated !!!</p>