How do you find duplicated elements of a vector that outputs the integer appearance of that duplicate instead of a logical?

Question

We know that the duplicated() function outputs a vector of logicals. What I want, however, is a vector of integers such that, if this is the (n+1)th time that this particular el

Answer 1

Here's a possible solution using base R (not sure if vectorized enough for you)

as.numeric(ave(x, x, FUN = seq)) - 1L
## [1] 0 0 1 0 1 2 1 3 4 2

Or something similar using data.table package

library(data.table)
as.data.table(x)[, y := seq_len(.N) - 1L, by = x][]
#     x y
#  1: c 0
#  2: b 0
#  3: b 1
#  4: a 0
#  5: a 1
#  6: b 2
#  7: c 1
#  8: b 3
#  9: b 4
# 10: c 2

Or maybe a dplyr option

library(dplyr)
data.frame(x) %>%
  group_by(x) %>%
  mutate(y = row_number() - 1L)
# Source: local data frame [10 x 2]
# Groups: x
# 
#    x y
# 1  c 0
# 2  b 0
# 3  b 1
# 4  a 0
# 5  a 1
# 6  b 2
# 7  c 1
# 8  b 3
# 9  b 4
# 10 c 2