Question

foo <- c("a","a",NA,"b","a","a","b","b")

How to replace “b” by whatever if previous element is NA ?

foo[foo=="b" & "previous-element"==NA] <- "whatever"

So expected output would be :

result <- c("a","a",NA,"whatever","a","a","b","b")

So only “b” (many in real datas) preceded by NA would be changed.

Thank’s for helping !

Answer 1

A simple solution:

foo[-1][foo[-1] == "b" & is.na(head(foo, -1))] <- "whatever"

---

Update:

A solution with rollapply from the zoo package:

library(zoo)
foo[-1][rollapply(foo, 2, identical, c(NA, "b"))] <- "whatever"