Creating row identifier within a group

Question

I have data in the following form. There are more than a million rows. I just want to create another column which helps me in row identification of grouped Item3 . The first two columns are irrelevant. Just added to let know that I have other columns in the dataset. I used cumsum and group_indices but didn't work.

Item1	Item2	Item3
One	Two	A
One	Two	A
One	Two	A
One	Two	B
One	Two	B
One	Two	C

Item1	Item2	Item3	Identifier
One	Two	A	1
One	Two	A	2
One	Two	A	3
One	Two	B	1
One	Two	B	2
One	Two	C	1

Answer 1

library(tidyverse)

data <- tibble(
  Item1 = c("One", "One", "One", "One", "One", "One"),
  Item2 = c("Two", "Two", "Two", "Two", "Two", "Two"),
  Item3 = c("A", "A", "A", "B", "B", "C")
)

data %>% 
  mutate(ID = row_number(), .by = Item3))

  Item1 Item2 Item3     ID
  <chr> <chr> <chr> <int>
1 One   Two   A         1
2 One   Two   A         2
3 One   Two   A         3
4 One   Two   B         1
5 One   Two   B         2
6 One   Two   C         1

Credit to Chamkrai for the .by = Item3 idea 😃