How to alter a variable in every row after a condition has been met-CodePudding

Essentially, I need to alter every row that occurs after a certain condition has been met. Though I also need the loop to obey a grouping variable. A simplified version of my data (shown below), is the grouping variable (Groups), followed by a value (N) and then the conditional variable (R). You can create a simplified version of my data as follows:

Groups <- c("A", "A", "A", "A", "B", "B", "B", "B", "C", "C")
N <- c(1,1,1,1,1,1,1,1,1,1)
R <- c("N", "N", "Y", "N", "N", "N", "Y", "N", "N", "N")
Dat <- as.data.frame(cbind(Groups, N, R))

What I need, is for when R == "Y", that row and every row after that for that group, has 1 added to the N variable. So the solution should look like this:

   Groups N R
1       A 1 N
2       A 1 N
3       A 2 Y
4       A 2 N
5       B 1 N
6       B 1 N
7       B 2 Y
8       B 2 N
9       C 1 N
10      C 1 N

So the loop needs to restart with each new group. Ideally, a solution within dplyr is preferred but I have not been able to find one yet.

Any help or guidance would be much appreciated!

CodePudding user response：

Do a group by cumsum on a logical vector and add to the 'N'

library(dplyr)
Dat %>% 
   group_by(Groups) %>% 
   mutate(N = cumsum(R == "Y")   N) %>%
   ungroup()

-output

# A tibble: 10 × 3
   Groups     N R    
   <chr>  <dbl> <chr>
 1 A          1 N    
 2 A          1 N    
 3 A          2 Y    
 4 A          2 N    
 5 B          1 N    
 6 B          1 N    
 7 B          2 Y    
 8 B          2 N    
 9 C          1 N    
10 C          1 N

data

Dat <- data.frame(Groups, N, R)
# NOTE: Using `cbind` converts to `matrix` and matrix can have only a single class.  Directly use `data.frame` instead of roundabout way which is not a correct approach.