title: Sum values in rows with same names in R
body: <p>I have a data frame with some rows that are named the same as this (there are many more names other than those shown and many more columns wich names are variable):</p>
  <pre><code>          category1   category2 
  bacteria      5           6
  bacteria      3           6
  bacteria      7           4
  archaea       1           2
  archaea       3           7
  </code></pre>
  <p>I need to sum the values of the rows that have the same to get an output like this:</p>
  <pre><code>          category1   category2 
  bacteria      15          16
  archaea       4           9
  </code></pre>