cumsum by group

Question

Suppose data looks like

group1 group2 num
A      sg     1
A      sh     2
A      sg     4
B      at     3
B      al     7

a <- cum

Answer 1

library(data.table)

data <- data.table(group1=c('A','A','A','B','B'),sum=c(1,2,4,3,7))

data[,list(cumsum = cumsum(sum)),by=list(group1)]