How to set a value for each row referenced in a single case

Here's some example code.

df <- structure(list(v1 = c(1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 
1, 1, 1, 1, 0, 1, 0, 0, 1), v2 = c(1, 0, 1, 1, 0, 1, 0, 1, 0, 
1, 0, 1, 0, 1, 0, 0, 0, 1, 1, 1, 0, 1), flag = c(NA, NA, NA, 
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, 
NA, NA, NA)), class = "data.frame", row.names = c(NA, -22L))

I am interested in coding the variable "flag" such that when v1 = 0 and the next v2 = 0, both rows get a 'flag' in the flag column. If a flag has already been placed, it cannot be changed (i.e., row 5 would not be flagged alone, but was already flagged when looking at row 4)

Here is the desired dataframe.

df2 <- structure(list(v1 = c(1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 
1, 1, 1, 1, 0, 1, 0, 0, 1), v2 = c(1, 0, 1, 1, 0, 1, 0, 1, 0, 
1, 0, 1, 0, 1, 0, 0, 0, 1, 1, 1, 0, 1), flag = structure(c(NA, 
NA, NA, 1L, 1L, 1L, 1L, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, 
NA, NA, 1L, 1L, NA), .Label = "flag", class = "factor")), class = "data.frame", row.names = c(NA, 
-22L))

I have started with the code below, which matches the conditions I would like, but only changes the row matching the v1 condition, not both.

df2 <- df %>% 
  mutate( flag = case_when(v1 == 0 & lead(v2)== 0 ~ 'flag'))

This is a very simplified version of my true data and I know there are options other than using case_when, but I would really like to use the case_when. function for this (I would also be open to using ifelse.

CodePudding user response：

library(tidyverse)
df %>%
  mutate(f = v1 == 0 & lead(v2) == 0,
      flag = ifelse(f|lag(f), 'flag', NA), f = NULL)

   v1 v2 flag
1   1  1 <NA>
2   1  0 <NA>
3   0  1 <NA>
4   0  1 flag
5   0  0 flag
6   0  1 flag
7   1  0 flag
8   1  1 <NA>
9   0  0 <NA>
10  1  1 <NA>
11  0  0 <NA>
12  1  1 <NA>
13  0  0 <NA>
14  1  1 <NA>
15  1  0 <NA>
16  1  0 <NA>
17  1  0 <NA>
18  0  1 <NA>
19  1  1 <NA>
20  0  1 flag
21  0  0 flag
22  1  1 <NA>