title: Manipulate data frame columns
body: <p>I have a data frame in the format of:</p>
  
  <pre><code>Col A   ColB   ColC  
  "Text1" 30     50
  "Text2" 100    120
  "Text3" 500    560
  "Text4" 705    800
  </code></pre>
  
  <p>And so on...</p>
  
  <p>I want to achieve a data frame that looks like:</p>
  
  <pre><code>Col A   ColB   ColC  ColD
  "Text1" 30     50    99
  "Text2" 100    120   499
  "Text3" 500    560   704
  "Text4" 705    800   ...(Text5's position -1)
  </code></pre>
  
  <p>I basically want to get the following texts "ColB-1" inserted into the previous text's ColD. I was wondering how I could do so?</p>