title: Summing a column based on other columns values
body: <p>I would like to sum values of one column based on another column(s) value as efficiently as possible.  I was not sure if there was a way to use the summarize command.  Here is an example data set:</p>
  
  <pre><code>Cancer1   Cancer2   Cancer3   Disease1
  1         0         1         1
  0         1         0         0
  1         0         0         1 
  </code></pre>
  
  <p>In this case I am looking to sum Disease1 based on if the person has a given cancer. I am looking for an output that would say the total number of people that have Cancer1 and Disease1 is 2, the total number of people that have Cancer2 and Disease1 is 0 and the total number of people that have Cancer3 and Disease1 is 1.</p>