How to use R to replace missing values with the sum of previous 4 values in a column?

Question

I have a dataframe that contains (among other things) three columns that have missing values every 5 rows. These missing values need to be replaced with the sum of the previous 4 values in their respective column.

For example, let's say my dataframe looked like this:

id        category1 category2 category3
123         5        10        10
123         6        11        15
123         6        12        23
123         4        10         6
123        NA        NA        NA
567        24        17        15

Those NAs need to represent a "total" based on the sum of the previous 4 values in their column, and this needs to repeat throughout the entire dataframe because the NAs occur every 5 rows. For instance, the three NAs in the mock example above should be replaced with 21, 43, and 54. 5 rows later, the same process will need to be repeated. How can I achieve this?

Answer 1

Another possible solution:

library(dplyr)

df %>% 
  group_by(id) %>% 
  mutate(across(everything(), ~ if_else(is.na(.x), sum(.x, na.rm = T), .x))) %>% 
  ungroup

#> # A tibble: 6 × 4
#>      id category1 category2 category3
#>   <int>     <int>     <int>     <int>
#> 1   123         5        10        10
#> 2   123         6        11        15
#> 3   123         6        12        23
#> 4   123         4        10         6
#> 5   123        21        43        54
#> 6   567        24        17        15