Create id sequence for flag with specific condition

Question

I need create a id sequence for a specific condition: flag==1, an example of my problem is:

library(dplyr)
set.seed(123)
a <- data.frame(id = 1:10,
                flag = rbinom(10,1,0.2))
print(a)

   id flag
1   1    0
2   2    0
3   3    0
4   4    1
5   5    1
6   6    0
7   7    0
8   8    1
9   9    0
10 10    0

I tried to create a id variable only for flag==1 but lamentably also identify flag==0, an example of my command result and desire result is :

a %>%
mutate(try_seq = cumsum(c(TRUE, diff(flag) != 0)))

>  id flag try_seq desire_seq
1   1    0       1      NA
2   2    0       1      NA    
3   3    0       1      NA
4   4    1       2      1
5   5    1       2      1
6   6    0       3      NA
7   7    0       3      NA
8   8    1       4      2
9   9    0       5      NA
10 10    0       5      NA
10 10    0       5      NA

Answer 1

This will return the result you are looking for:

cumsum(c(a$flag[1], diff(a$flag)) > 0) * NA^!a$flag
 [1] NA NA NA  1  1 NA NA  2 NA NA

The NA^a$flag trick uses the idea that any value raised to the 0th power is 1. Otherwise, we use diff to check for a positive change in variable.