title: Order a "mixed" vector (numbers with letters)
body: <p>How can I order a vector like </p>
  
  <pre><code>c("7","10a","10b","10c","8","9","11c","11b","11a","12") -&gt; alph
  </code></pre>
  
  <p>in</p>
  
  <pre><code>alph
  [1] "7","8","9","10a","10b","10c","11a","11b","11c","12"
  </code></pre>
  
  <p>and use it to sort a data.frame, like</p>
  
  <pre><code>V1 &lt;- c("A","A","B","B","C","C","D","D","E","E")
  V2 &lt;- 2:1 
  V3 &lt;- alph
  df &lt;- data.frame(V1,V2,V3)
  </code></pre>
  
  <p>and order the row to obtain (order V2 and then V3)</p>
  
  <pre><code> V1 V2  V3
  C  1   9
  A  1 10a
  B  1 10c
  D  1 11b
  E  1  12
  A  2   7
  C  2   8
  B  2 10b
  E  2 11a
  D  2 11c
  </code></pre>