title: Multiply column values per group in r
body: <p>I have a data frame as following:</p>
  
  <pre><code> id     weight     value
  231      50         0.6
  231      50         0.43
  420      30         0.86
  420      30         0.12
  </code></pre>
  
  <p>How can I multiply all values per each id and weight to have the following table:</p>
  
  <pre><code>id     weight     value
  231     50       0.6*0.43
  420     30       0.86*0.12
  </code></pre>