How to tally occurrences of a value in a column in R?

Question

I have the following data:

df <- data.frame(id = c("1", "1", "1", "1", "2", "2", "2"), 
                 x = c(0, 1, 0, 1, 0, 1, 0))

id x
 1 0
 1 1
 1 0
 1 1
 2 0
 2 1
 2 0

I want to tally the occurences of 1 in x. The desired output:

id x count
 1 0     0
 1 1     1
 1 0     0
 1 1     2
 2 0     0
 2 1     1
 2 0     0

How do I do this? A dplyr solution is preferred.

Answer 1

You may take cumulative sum of x values and keep the result to be 0 where value of x is 0 for each id.

library(dplyr)

df %>%
  mutate(count = replace(cumsum(x), x == 0, 0), .by = id)

#  id x count
#1  1 0     0
#2  1 1     1
#3  1 0     0
#4  1 1     2
#5  2 0     0
#6  2 1     1
#7  2 0     0