title: Change column values depending a another column value in pandas
body: <p>I have a dataframe in <code>python</code> such as :</p>
  
  <pre><code>seqnames    start   end name    number  strand
       A       50     453   A      1        -
       B       30     322   A      2        -
       C       10     432   A      3        -
       D       36     344   A      4        +
       E       40     321   A      5        +
       F       78     234   A      6        -
  </code></pre>
  
  <p>and I would like to change de values in the start and end columns depending on the symbole in the <code>strand column</code>.</p>
  
  <p>So for each line, if the <code>strand</code> is <code>-</code> than do <code>start+1</code> and <code>end-2</code> 
  if the strand is <code>+</code> than do nothing</p>
  
  <p>here I should get : </p>
  
  <pre><code>seqnames    start   end name    number  strand
  A   51  451 A   1   -
  B   31  320 A   2   -
  C   11  430 A   3   -
  D   36  344 A   4   +
  E   40  321 A   5   +
  F   79  232 A   6   -
  </code></pre>
  
  <p>Thank you for your help </p>