How do I create a column in R that shows a cumulative amount over time?

I have a dataset of three variables: identifier, date, and amount. I want to add a fourth column that shows the cumulative amount over time. For example:

In Excel, I’d use a SUMIF formula to create this fourth column, but I honestly have no clue how to do this in R. Can someone help?

>Solution :

For a dplyr solution you can use mutate to add a column with the cumulative sum for each group.

    library(dplyr)
    
    df <- tibble(identifier = c(rep('aaa', 3), rep('bbb', 4)),
                 date = c(seq(1,3,1),seq(1,4,1)),
                 amount = c(5,6,8,5,4,0,11))


# A tibble: 7 x 3
  identifier  date amount
  <chr>      <dbl>  <dbl>
1 aaa            1      5
2 aaa            2      6
3 aaa            3      8
4 bbb            1      5
5 bbb            2      4
6 bbb            3      0
7 bbb            4     11
    
    df %>%
      group_by(identifier) %>%
      mutate('cumulative amount' = cumsum(amount))

# A tibble: 7 x 4
# Groups:   identifier [2]
  identifier  date amount `cumulative amount`
  <chr>      <dbl>  <dbl>               <dbl>
1 aaa            1      5                   5
2 aaa            2      6                  11
3 aaa            3      8                  19
4 bbb            1      5                   5
5 bbb            2      4                   9
6 bbb            3      0                   9
7 bbb            4     11                  20