title: Using Reduce function to merge recursively
body: <p>If I have a list of a list, and the list contains a set of dataframes and I want to merge the dataframes together but don't to merge all the list together. For example</p>
  
  <pre><code> list&lt;- list(list(df1_2010,df2_2010,df3_2010), list(df1_2011,df2_2011,df3_2011), list(df1_2012,df2_2012,df3_2012))
  </code></pre>
  
  <p>And i want to merge all the 2010 dataframe together by let say column id. And I want to merge the 2011 dataframes together by a similar column id, and I want to merge all the 2012 dataframes together by another similar column id.</p>
  
  <p>I want to output a list of merged dataframes by year:</p>
  
  <pre><code>   list(df2010, df2011, df2012)
  </code></pre>
  
  <p>Here's a schematic of how I want to use the Reduce function:</p>
  
  <pre><code>   f&lt;-function(b) merge(...,by="ID",all.x=T)
     list&lt;- Reduce(f, list)
  </code></pre>
  
  <p>But I think this will merge all three lists together instead of each list separately. Let me know  your suggestions.</p>