title: Aggregate shared values into a new column
body: <p>I'm trying to aggregate the values for several rows into a new column. Specifically, I'm looking to sum Conflict_Events for the same Year_Week and Country into a new column Conflicts_Week. For example:</p>
  <p><a href="https://i.stack.imgur.com/V5zUT.png" rel="nofollow noreferrer">enter image description here</a></p>
  <p>So Afghanistan's event count (Conflicts_Week) for 1989_4 would be 11. Armenia's for 1989_2 would be 3. I found something similar on stack, but it only matches using one variable instead of two: aggregate(x = df$Conflict_Events, by = list(df$Year_Week), FUN = sum). How would I include an additional variable, so that it matches on Country as well?</p>
  <p>Thank you in advance!</p>