How to flag the values of a column based on values of another column in R by using rollapply?

Question

I have a data frame, like this:

df <- data.frame (T  = c(1:20), L = c(1,2,9,4,6,8,3,4,2,5,2,5,9,10,3,5,1,1,2,2))

I want to Flag a T value Ti (and also Ti−1 and Ti+1) if Li is bigger than 6 (i.e. 3 values in total are flagged then).

How to do it by Rollapply?

Answer 1

Using rollapply -

library(dplyr)
library(zoo)

df %>%
  mutate(flag = rollapply(L > 6, 3, any, fill = FALSE))

#    T  L  flag
#1   1  1 FALSE
#2   2  2  TRUE
#3   3  9  TRUE
#4   4  4  TRUE
#5   5  6  TRUE
#6   6  8  TRUE
#7   7  3  TRUE
#8   8  4 FALSE
#9   9  2 FALSE
#10 10  5 FALSE
#11 11  2 FALSE
#12 12  5  TRUE
#13 13  9  TRUE
#14 14 10  TRUE
#15 15  3  TRUE
#16 16  5 FALSE
#17 17  1 FALSE
#18 18  1 FALSE
#19 19  2 FALSE
#20 20  2 FALSE