Group cumulative reduce

reduce.R

# https://stackoverflow.com/questions/40412516/how-to-write-a-cumulative-calculation-in-data-table

library(dplyr)
library(data.table)

dt <- tribble(
  ~"a", ~"b", ~"g",
  0.03, 3340, "B",
  0.03, NA, "B",
  0, NA, "B",
  0.05, NA, "B",
  0, NA, "B",
  0.03, 3502, "D",
  0.03, NA, "D",
  0, NA, "D",
  0.05, NA, "D",
  0.02, NA, "D"
)
setDT(dt)

dt[,
  z := Reduce(
    f = function(v, i) round(v / sum(1, a[i], na.rm = TRUE)),
    x = seq_len(.N)[-1],
    init = b[1],
    accumulate = TRUE
  ),
  by = .(g)
]