title: How to convert a list into a matrix in R
body: <p>I do have a list of vectors, like this:</p>
  
  <pre><code>[[1]]
  [1]  1  2  7  9 10 13 14 15 20
  
  [[2]]
  [1] 3 4 5 6
  
  [[3]]
  [1]  8 11 12
  
  [[4]]
  [1] 16 17 18 19
  
  [[5]]
  [1] 21 22 23
  </code></pre>
  
  <p>I want to convert the list to a matrix where the contents of each vector are associated with the list index number in double brackets.</p>
  
  <p>Example:</p>
  
  <pre><code>       [,1] [,2]
   [1,]   1    1
   [2,]   1    2
   [3,]   1    7
   [4,]   1    9
   [5,]   1   10
   [6,]   1   13
   [7,]   1   14
   [8,]   1   15
   [9,]   1   20
  [10,]   2    3
  [11,]   2    4
  [12,]   2    5
  [13,]   2    6
  [14,]   3    8
  [15,]   3   11
  [16,]   3   12
  [17,]   4   16
  [18,]   4   17
  [19,]   4   18
  [20,]   4   19
  [21,]   5   21
  [22,]   5   22
  [23,]   5   23
  </code></pre>