title: How to collect unique values, and sum across other columns with conditions
body: <p>I have a lot of financial trading data with around a million rows and I want to be able to condense this into a new data frame with a list of Unique UserIDs. I then want to be able to add up the &quot;trades&quot; for their account, with some conditions, ie if TransactionTypeId == 2 &amp; AC_Type== 19. I would use a sumifs in excel for this but the size of the file means its pretty much impossible to run on my computer.</p>
  <pre><code>df&lt;- structure(list(UserId = c(1, 1, 1, 1, 2, 
  2, 2, 3, 3, 3, 4, 5, 6, 
  6, 6, 7, 7, 7, 8, 8, 8, 
  8, 8, 9, 9, 9, 10, 11, 12, 
  12, 13, 13, 13, 14, 14, 15, 15, 
  16, 16, 16), TransactionTypeId = c(14, 1, 1, 70, 
  15, 1, 1, 14, 1, 1, 70, 14, 14, 1, 1, 14, 1, 1, 14, 1, 1, 1, 
  1, 14, 1, 1, 14, 14, 1, 1, 14, 1, 1, 1, 1, 70, 70, 14, 1, 1), 
      AC_Type = c(21, 21, 21, 21, 19, 19, 19, 19, 19, 19, 19, 19, 
      19, 19, 19, 21, 21, 21, 19, 19, 19, 19, 19, 19, 19, 19, 20, 
      19, 19, 19, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20), Trades = c(30, 
      30, 0.00067116, 0.00067115, 249, 249, 0.00533033, 48.75, 
      48.75, 0.00101298, 0.00533, 24.37, 146.25, 146.25, 0.00309109, 
      100.01, 100.01, 0.00233551, 97.5, 90, 0.00189134, 5, 0.00245851, 
      234, 234, 0.00500802, 100.01, 48.75, 48.5, 0.0275474, 24, 
      24, 0.00051975, 100, 0.00223998, 0.00051975, 0.00205, 9.75, 
      8.75, 0.00017811)), row.names = c(NA, -40L), class = c(&quot;tbl_df&quot;, 
  &quot;tbl&quot;, &quot;data.frame&quot;))
  </code></pre>