title: How to use R to create a word co-occurrence matrix
body: <p>I am a newbie in r.
  I have a set of data about online videos and their tags.
  The data looks like</p>
  
  <pre><code>film  tag1 tag2 tag3 tag4....
  1      A    B    C    D
  2      A    C    F    G 
  3      B    D    C    X 
  </code></pre>
  
  <p>I want to create a matrix which tells me the co-occurrence of the tags, such as: </p>
  
  <pre><code>       A    B   C    D .....
  A     10    13
  B     15    2
  C      3    16
  D     9     20
  </code></pre>
  
  <p>How should I do it?</p>