title: convert numbers to letters in a data.frame
body: <p>In R, I have some matrices of numbers and I am going to convert each number to its equivalent letter; 1 to "a", 2 to "b", and so on.</p>
  
  <p>Assume this is the matrix:</p>
  
  <pre><code>set.seed(1)
  x &lt;- data.frame(matrix(sample(1:16,12,T),nrow=3,ncol=4))
  #  X1 X2 X3 X4
  #1  5 15 16  1
  #2  6  4 11  4
  #3 10 15 11  3
  </code></pre>
  
  <p>This is the expected output:</p>
  
  <pre><code>#  X1 X2 X3 X4
  #1  e  o  p  a
  #2  f  d  k  d
  #3  j  o  k  c
  </code></pre>
  
  <p>I used <code>letters[x]</code> and <code>letters[list(x)]</code> but it gave this </p>
  
  <blockquote>
    <p>Error in letters[list(x)] : invalid subscript type 'list'</p>
  </blockquote>