title: create two-way table value with data R
body: <p>I have a data frame with 2 groups and values, and I would like to have a two way table with Values, but with the function "table" I only have the number of occurrences as values.
  Example :</p>
  
  <p>I have a dataframe like this :</p>
  
  <pre><code> GroupA   GroupB   Value
   A1       B1       4
   A1       B2       3
   A2       B1       2
   A1       B1       3
  </code></pre>
  
  <p>And I would like a table like this :</p>
  
  <pre><code>    A1     A2
  B1  7      2
  B2  3      0
  </code></pre>
  
  <p>(Since there is no data for A2 B2, it is 0)</p>
  
  <p>Any idea how I could do this?</p>