Comparing a column value to all its leading values within dplyr::mutate

Question

I have data consisting of a single vector / column in a tibble:

my_tibble <- tibble(score = c(1,2,3,4,9,8,7,6,5,4))

For every row of my_tibble$score I want to calculate the difference to the largest "leading" element in the same column. This new column shall be called "difference". For example, the first row of difference should be 1 - 9, while the fifth row should be 9 - 8, and the last row will turn NA, as there is no value coming behind/below the 4.

In the end, the new tibble should look like this:

score | difference
<dbl>   <dbl>
1         -8
2         -7
3         -6
4         -5
9          1
8          1
7          1
6          1
5          1
4         NA

I want to achieve this using dplyr and have so far tried many variations of mutate like

my_tibble %>%
 mutate(difference = score[which(score > score)])

Hoping to find some way that the second "score" in the which funtction refers to the current row being mutated. However I was unsuccessfull after hours of trying and desperately searching for a solution online.

The closest I found was dplyr: Summing n leading values, however that still leaves me with the problem that I want the difference to the maximum leading value of all leading values, not only of the closest n leading values.

Help and/or referral to whereever this has been answered or addressed before is greatly appreciated!

Answer 1

My solution:

my_tibble <- my_tibble %>%
 mutate(difference = map_dbl(seq_along(score), function(y) ifelse(y == length(score),
 NA_real_, score[y] - max(c(score[-(1:y)])))))

Output

> my_tibble
# A tibble: 10 x 2
   score difference
   <dbl>      <dbl>
 1     1         -8
 2     2         -7
 3     3         -6
 4     4         -5
 5     9          1
 6     8          1
 7     7          1
 8     6          1
 9     5          1
10     4         NA