title: Need to sum rows of data across multiple columns of variables using dplyr
body: <p>Others have asked similar questions, but their data structure was a bit different. My data set has multiple columns for both grouping variables and numeric data. I need to sum the numeric data per row and output the sum to a new data column. See example <code>DATA</code> set and the desired <code>RESULTS</code> tables below. I prefer to find a solution using the <code>mutate</code> function in <code>dplyr</code>. I primary use the <code>dplyr</code> package to manipulate my data sets. I can accomplish this task via <code>gather</code>, <code>group_by</code>, and <code>sumarise</code> functions in <code>dplyr</code>, but I am working with very large data sets which can cause “gathered” data tables to be in excess of 2,000,000 rows.  Thanks in advance.  </p>
  
  <pre><code>DATA = data.frame(SITE = c("A","A","A","A","B","B","B","C","C"), 
                    DATE = c("1","1","2","2","3","3","3","4","4"), 
                    STUFF = c(1, 2, 30, 40, 100, 200, 300, 5000, 6000),
                    STUFF2 = c(2, 4, 60, 80, 200, 400, 600, 10000, 12000))
  
  RESULT = data.frame(SITE = c("A","A","A","A","B","B","B","C","C"), 
                      DATE = c("1","1","2","2","3","3","3","4","4"), 
                      STUFF = c(1, 2, 30, 40, 100, 200, 300, 5000, 6000),
                      STUFF2 = c(2, 4, 60, 80, 200, 400, 600, 10000, 12000),
                      SUM_STUFF = c(3, 6, 90, 120, 300, 600, 900, 15000, 18000))
  </code></pre>