code data
1     A    1
2          2
3          3
4          4
5          5
6          6
7          7
8          8
9          9
10        10
11    B   11
12        12
13        13
14        14
15        15
16    C   16
17        17
18        18
19        19
20        20

code data
1     A    1
2     A    2
3     A    3
4     A    4
5     A    5
6     A    6
7     A    7
8     A    8
9     A    9
10    A   10
11    B   11
12    B   12
13    B   13
14    B   14
15    B   15
16    C   16
17    C   17
18    C   18
19    C   19
20    C   20

# Create mock data frame
df <- data.frame(code = c("A", rep("", 9),
                          "B", rep("", 4),
                          "C", rep("", 4)),
                 data = 1:20)

# For loop over rows (BAD!)
for (i in seq(2, nrow(df))) {
  df[i,]$code <- ifelse(df[i,]$code == "", df[i-1,]$code, df[i, ]$code)
}

df %>%
  mutate(code = na_if(code,"")) %>%
  fill(code)

   code data
1     A    1
2     A    2
3     A    3
4     A    4
5     A    5
6     A    6
7     A    7
8     A    8
9     A    9
10    A   10
11    B   11
12    B   12
13    B   13
14    B   14
15    B   15
16    C   16
17    C   17
18    C   18
19    C   19
20    C   20