title: How to sum a variable by group
body: <p>I have a data frame with two columns. First column contains categories such as "First", "Second", "Third", and the second column has numbers that represent the number of times I saw the specific groups from "Category".</p>
  
  <p>For example:</p>
  
  <pre><code>Category     Frequency
  First        10
  First        15
  First        5
  Second       2
  Third        14
  Third        20
  Second       3
  </code></pre>
  
  <p>I want to sort the data by Category and sum all the Frequencies:</p>
  
  <pre><code>Category     Frequency
  First        30
  Second       5
  Third        34
  </code></pre>
  
  <p>How would I do this in R?</p>