How to output duplicated rows

Question

I have the following data:

x1  x2  x3  x4
34  14  45  53 
2   8   18  17
34  14  45  20
19  78  21  48 
2   8   18  5

In rows 1 and 3; and

Answer 1

There is another way to solve both questions using two packages.

library(DescTools)
library(dplyr)
dat[AllDuplicated(dat[1:3]), ] %>% # this line is to find duplicates
  group_by(x1, x2) %>% # the lines followed are to sum up
  mutate(x4 = sum(x4)) %>%
  unique()
# Source: local data frame [2 x 4]
# Groups: x1, x2
# 
#   x1 x2 x3 x4
# 1 34 14 45 73
# 2  2  8 18 22