How to get the cumulative sum by group in R?

Question

Suppose I have a dataframe such that:

df<-data.frame(id=1:8,group=c(1,0,0,1,1,0,1,0),rep=c(rep(\"d1\",4),rep(\"d2\",4)),value=rbinom(8,1,0.6))
df
  id gro         


        

Answer 1

library(data.table)

# convert to data.table in place
setDT(df)

# dcast and do individual sums
dt.cast = dcast.data.table(df, group ~ rep, value.var = 'value',
                           fun.aggregate = sum)
dt.cast
#   group d1 d2
#1:     0  0  1
#2:     1  1  2

# cumsum
dt.cast[, as.list(cumsum(unlist(.SD))), by = group]
#   group d1 d2
#1:     0  0  1
#2:     1  1  3