title: How to group by two columns in R
body: <p>I have a data frame that I am trying to group and then sum based on two columns. The two columns are characters with one being month and the other variable.</p>
  
  <p>The following is a sample of the data frame and structure.</p>
  
  <pre><code>#row.names   month    variable   amount
    1          1-Jan       x        1000
    2          1-Jan       x        3000
    3          2-Feb       z        5000
    4          2-Feb       y        3000 
  </code></pre>
  
  <p>I tried to group the data first and then I was going to try to summarise, however I am unable to get group_by_() to do the trick. Below is the code I tried.</p>
  
  <pre><code>byVarMonth &lt;- group_by_(df, variable, (as.date(month)))
  </code></pre>
  
  <p>Thanks for the help.</p>