Need to sum rows of data across multiple columns of variables using dplyr

Question

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 DATA set and the desired RESULTS tables below. I prefer to find a solution using the mutate function in dplyr . I primary use the dplyr package to manipulate my data sets. I can accomplish this task via gather , group_by , and sumarise functions in dplyr , 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.

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))

Answer 1

This looks like it meets your needs:

RESULT <- DATA %>%
  rowwise() %>%
  mutate(SUM_STUFF = sum(STUFF,STUFF2))

The key is to use rowwise (which may have been added to to a version of dplyr after you asked your question).

> RESULT
Source: local data frame [9 x 5]
Groups: <by row>

    SITE   DATE STUFF STUFF2 SUM_STUFF
  (fctr) (fctr) (dbl)  (dbl)     (dbl)
1      A      1     1      2         3
2      A      1     2      4         6
3      A      2    30     60        90
4      A      2    40     80       120
5      B      3   100    200       300
6      B      3   200    400       600
7      B      3   300    600       900
8      C      4  5000  10000     15000
9      C      4  6000  12000     18000