Cumulative sum by group in sqldf?

Question

I have a data frame with 3 variables: place, time, and value (P, T, X). I want to create a fourth variable which will be the cumulative sum of X. Normally I like to do group

Answer 1

Or, another option is data.table.

> library(data.table)
> DT = data.table(place = 1:4, time = rep(1:3, each = 4), value = 1:3)
> setkey(DT,place,time)   # order by place and time
> DT
      place time value
 [1,]     1    1     1
 [2,]     1    2     2
 [3,]     1    3     3
 [4,]     2    1     2
 [5,]     2    2     3
 [6,]     2    3     1
 [7,]     3    1     3
 [8,]     3    2     1
 [9,]     3    3     2
[10,]     4    1     1
[11,]     4    2     2
[12,]     4    3     3
> DT[,list(time,value,cumsum(value)),by=place]
      place time value V3
 [1,]     1    1     1  1
 [2,]     1    2     2  3
 [3,]     1    3     3  6
 [4,]     2    1     2  2
 [5,]     2    2     3  5
 [6,]     2    3     1  6
 [7,]     3    1     3  3
 [8,]     3    2     1  4
 [9,]     3    3     2  6
[10,]     4    1     1  1
[11,]     4    2     2  3
[12,]     4    3     3  6
>