Loop to sum up observation greater than subject in R

Question

I have a data set that looks like that

set.seed(100)
da <- data.frame(exp = c(rep(\"A\", 4), rep(\"B\", 4)), diam = runif(8, 10, 30))

Fo

Answer 1

If you don't require the initial order in the result you could do it quite efficiently like this:

library(data.table)
setorder(setDT(da), exp, -diam)
da[, d2 := cumsum(diam) - diam, by = exp]

da
#   exp     diam       d2
#1:   A 21.04645  0.00000
#2:   A 16.15532 21.04645
#3:   A 15.15345 37.20177
#4:   A 11.12766 52.35522
#5:   B 26.24805  0.00000
#6:   B 19.67541 26.24805
#7:   B 19.37099 45.92347
#8:   B 17.40641 65.29445

Using dplyr, that would be:

library(dplyr)
da %>%
  arrange(exp, desc(diam)) %>%
  group_by(exp) %>%
  mutate(d2 = cumsum(diam) - diam)