Q:

R cummax function with NA

data   
data=data.frame("person"=c(1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2),
                 "score"=c(1,2,1,2,3,1,3,NA,4,2,1,NA,2,NA,3,1,2,4),
                  "want"=c(1,2,1,2,3,3,3,3,4,2,1,1,2,2,3,3,3,4))

attempt
library(dplyr)

data = data %>%
  group_by(person) %>%
  mutate(wantTEST = ifelse(score >= 3 | (row_number() >= which.max(score == 3)), 
                        cummax(score), score), 
         wantTEST = replace(wantTEST, duplicated(wantTEST == 4) & wantTEST == 4, NA))

i am basically working to use the cummax function but only under specific circumstances.  i want to keep any values (1-2-1-1) except if there is a 3 or 4 (1-2-1-3-2-1-4) should be (1-2-1-3-3-4).  if there is NA value i want to carry forward previous value.  thank you. 

A:

Here's one way with tidyverse.  You may want to use fill() after group_by() but that's somewhat unclear. 
data %>% 
  fill(score) %>% 
  group_by(person) %>% 
  mutate(
    w = ifelse(cummax(score) > 2, cummax(score), score)
  ) %>%
  ungroup()

# A tibble: 18 x 4
   person score  want     w
    <dbl> <dbl> <dbl> <dbl>
 1      1     1     1     1
 2      1     2     2     2
 3      1     1     1     1
 4      1     2     2     2
 5      1     3     3     3
 6      1     1     3     3
 7      1     3     3     3
 8      1     3     3     3
 9      1     4     4     4
10      2     2     2     2
11      2     1     1     1
12      2     1     1     1
13      2     2     2     2
14      2     2     2     2
15      2     3     3     3
16      2     1     3     3
17      2     2     3     3
18      2     4     4     4

