title: Finding unique combinations in a dataset
body: <p>I have the following data set:</p>
  
  <pre><code>library(data.table)
  
  dt1 &lt;- data.table(Activity1 = c("A","A","B","B","C","C"),
                    Activity2 = c("B","C","A","C","A","B"),
                    participants = c(12, 22, 4, 20, 3, 31))
  
  # OUTPUT (dt1)
  # Activity1 Activity2 participants
  # 1:         A         B           12
  # 2:         A         C           22
  # 3:         B         A            4
  # 4:         B         C           20
  # 5:         C         A            3
  # 6:         C         B           31
  </code></pre>
  
  <p>I am attempting to find all the unique combinations of Activity1 and Activity2 such that I would end up with a dataset looking like the following (that sums the potential participants):</p>
  
  <pre><code># DESIRED Dataset
  #    Activity1 Activity2 participants
  # 1:         A         B           16
  # 2:         A         C           25
  # 4:         B         C           51
  </code></pre>
  
  <p>I am still relatively new to R and would appreciate any quidance.
  Thanks in advance.</p>