Suppose data looks like
group1 group2 num
A sg 1
A sh 2
A sg 4
B at 3
B al 7
a <- cumsum(data[,"num"]) # 1 3 7 10 17
I need something accumulated by groups. In reality,I have multiple columns as grouping indicators. I want to get the accumulated sum by the subgroup I define.
E.g
If I group by group1 only, then the output should be
group1 sum
A 1
A 3
A 7
B 3
B 10
If I group by two variables group1,group2 then the output is
group1 group2 sum
A sg 1
A sh 2
A sg 5
B at 3
B al 7
Answer
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)]