title: select a specific columns in R nested list
body: <p>suppose i have a list of data frames, just like this:</p>
  
  <pre><code>M1 &lt;- data.frame(matrix(1:4, nrow = 2, ncol = 2))
  M2 &lt;- data.frame(matrix(1:9, nrow = 3, ncol = 3))
  M3 &lt;- data.frame(matrix(1:4, nrow = 2, ncol = 2))
  
  mlist &lt;- list(M1, M2, M3)
  </code></pre>
  
  <p>and now i want to select X1 columns from all of dataframes, I tried :</p>
  
  <pre><code>M.X1 &lt;- mlist$X1
  </code></pre>
  
  <p>but failed with NULL:</p>
  
  <pre><code>&gt; mlist$X1
  NULL
  </code></pre>
  
  <p>I don't want to use <code>for</code> to extract each data frames' <code>X1</code>, is there some better way to do this ? And what if extract columns <code>X3</code> ? (which means some columns may not exists in other row)  </p>