Rのmutate()で、case_when中にfor文をいれたい

set.seed(0)
df <- tibble(POS = runif(10, 0, 1) * 10)
df <- df %>% 
  mutate(newcol = cut(POS,
                      breaks = c(0, 1, 2, 3, 4, 5, 6, 7, 10),
                      labels = c("A", NA, "B", NA, "C", NA, "D", NA),
                      include.lowest = TRUE))
df

# A tibble: 10 × 2
     POS newcol
   <dbl> <fct> 
 1  8.97 NA    
 2  2.66 B     
 3  3.72 NA    
 4  5.73 NA    
 5  9.08 NA    
 6  2.02 B     
 7  8.98 NA    
 8  9.45 NA    
 9  6.61 D     
10  6.29 D   

set.seed(0)
df <- tibble(POS = runif(10, 0, 20))
tbl_breaks <- tibble(
  START = c(2, 6, 10, 16),
  END = c(5, 9, 13, 19),
  ANSWER = c(1, 2, NA, 1)
)

breaks <- tbl_breaks %>%
  select(START, END) %>%
  t() %>%
  as.vector()
breaks <- c(0, breaks, 20)
breaks
labels <- tbl_breaks %>% 
  select(ANSWER) %>%
  mutate(DUMMY = NA) %>%
  t() %>%
  as.vector()
labels <- c(NA, labels)
labels

df <- df %>% 
  mutate(newcol = cut(POS,
                      breaks = breaks,
                      labels = labels,
                      include.lowest = TRUE))

df

# A tibble: 10 × 2
     POS newcol
   <dbl> <fct> 
 1 17.9  1     
 2  5.31 NA    
 3  7.44 2     
 4 11.5  NA    
 5 18.2  1     
 6  4.03 1     
 7 18.0  1     
 8 18.9  1     
 9 13.2  NA    
10 12.6  NA