If condition is met, go 6 rows above and select the value in R

Question

I am trying to do in R the following: In my dataset (md2, attached), if L_ID=1, go 6 rows above and create a new dataframe with the variable Hway. I use the following command:

spc <- data.frame(md$Hdwy[c(md2$L_ID[-6], 0) == 1])

Expecting to see:

> spc
44.46 40.00

It seems to work, however when I checked I noticed that it selects the value 1 row above instead of 6:

> spc
    40.1 40.00

Any ideas?

Answer 1

How about something like this?

library(dplyr)
spc <- md2 %>%
         mutate(Lag = lag(Hdwy, 6L)) %>%
         filter(L_ID==1) %>%
         pull(Lag)
spc
[1] 44.46 40.00

Or with base R:

spc <- md2$Hdwy[which(md2$L_ID == 1) - 6]
spc
[1] 44.46 40.00

Sample Data:

md2 <- structure(list(Hdwy = c(45.01, 45.03, 449, 44.46, 43.63, 425, 
41.36, 40.53, 40.1, 39.97, 39.98, 40, 40, 40, 40, 41.36, 40.53, 
40.1, 40, 40), L_ID = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 
0, 0, 0, 0, 0, 0, 1)), class = "data.frame", row.names = c(NA, 
-20L))