使用dplyr在组内的累计计数

Question

I am trying to create a column that contains a cumulative count of another column.

My data:

df <- data.frame(brand = c("A","B","C","A","A","B","A","A","B","C"))

And this is my expected output:

    |Brand |  Count  |
    |:-----|--------:|
    |A     |        1|
    |B     |        1|
    |C     |        1|
    |A     |        2|
    |A     |        3|
    |B     |        2|
    |A     |        4|
    |A     |        5|
    |B     |        3|
    |C     |        2|

I have tried cumsum but it doesn't accept strings or factors:

df %>%
  group_by(Brand) %>%
  mutate(Count = cumsum(Brand))

Edit: For bonus points it would be great if the solution could be used on database tables also (SQL Server)

Answer 1

We can create the column with rowid of 'brand'

library(dplyr)
library(data.table)
 df %>%
    mutate(Count = rowid(brand))

---

Or use a row_number after grouping by 'brand'

df %>%
    group_by(brand) %>%
    mutate(Count = row_number())

---

Or using data.table

library(data.table)
setDT(df)[, Count := rowid(brand)]