title: Pandas -- Map values from one column to another column
body: <p>Below I have a DataFrame showing how each vehicle (Vehicle_ID) will visit the different places (place_id) in a particular sequence (Pos_in_route).</p>
  
  <pre><code>place_id    Lat      Lon      Vehicle_ID    Pos_in_route
      0   51.4457678  -0.45613      0           0
      1   52.497911   -1.903832     0           1
      2   52.332395   -1.346753     0           2
      0   51.4457678  -0.45613      0           3
      0   51.4457678  -0.45613      1           0
      4   52.110728   -0.463547     1           1
      3   52.276323   -1.579845     1           2
      5   52.423667   -0.609697     1           3
      0   51.4457678  -0.45613      1           4
  </code></pre>
  
  <p>Now I am trying to add another two columns to show the GPS coordinates  of the prior place visited (prior_lat, prior_lon), according to the column (Pos_in_route). If there is no prior position of place it will be itself (i.e. place_id = 0) </p>
  
  <pre><code>    place_id    Lat      Lon      Vehicle_ID    Pos_in_route   prior_lat    prior_lon
          0   51.4457678  -0.45613    0           0              51.4457678   -0.45613
          1   52.497911   -1.903832   0           1              51.4457678   -0.45613
          2   52.332395   -1.346753   0           2              52.497911    -1.903832
          0   51.4457678  -0.45613    0           3              52.332395    -1.346753
          0   51.4457678  -0.45613    1           0              51.4457678   -0.45613
          4   52.110728   -0.463547   1           1              51.4457678   -0.45613
          3   52.276323   -1.579845   1           2              52.110728    -0.463547
          5   52.423667   -0.609697   1           3              52.276323    -1.579845
          0   51.4457678  -0.45613    1           4              52.423667    -0.609697
  </code></pre>
  
  <p>This is a small part of my program but it's very hard for me to show the complete codes. Maybe there is a simple way to add the last two columns at the end of my program?</p>