title: Merge many data frames from csv files, when ID column is implied?
body: <p>I'd like to merge a bunch of data frames together (because it seems many operations are easier if you're only dealing w/ one, but correct me if I'm wrong).</p>
  
  <p>Currently I have one data frame like this:</p>
  
  <pre><code>ID, var1, var2
  A,  2,    2
  B,  4,    5
  .
  .
  Z,  3,    2
  </code></pre>
  
  <p>Each ID is on a single row w/ several single measurements</p>
  
  <p>I also have a csv file w/ repeated measurement for each ID, like:</p>
  
  <p>filename = ID_B.csv</p>
  
  <pre><code>time, var4, var5
  0,    1,    2
  1,    4,    5
  2,    1,    6
  ...
  </code></pre>
  
  <p>What I'd like is:</p>
  
  <pre><code>ID, time, va1, var2, var4, var5
  ...
  B,  0,    4,   5,    1,    2,
  B,  1,    4,   5,    4,    5,
  B,  2,    4,   5,    1,    6,
  ...
  </code></pre>
  
  <p>I don't really care about the column order.  The only solution I can think of is to add the ID column to each csv file then loop through them calling <code>merge()</code> several times.  Is there a more elegant approach?</p>