r - Calculate cumulative sum (cumsum) by group

Question

With data frame:

df <- data.frame(id = rep(1:3, each = 5)
                 , hour = rep(1:5, 3)
                 , value = sample(1:15))

I want to add a cumulative sum column that matches the id:

df
   id hour value csum
1   1    1     7    7
2   1    2     9   16
3   1    3    15   31
4   1    4    11   42
5   1    5    14   56
6   2    1    10   10
7   2    2     2   12
8   2    3     5   17
9   2    4     6   23
10  2    5     4   27
11  3    1     1    1
12  3    2    13   14
13  3    3     8   22
14  3    4     3   25
15  3    5    12   37

How can I do this efficiently? Thanks!

Answer 1

df$csum <- ave(df$value, df$id, FUN=cumsum)

ave is the "go-to" function if you want a by-group vector of equal length to an existing vector and it can be computed from those sub vectors alone. If you need by-group processing based on multiple "parallel" values, the base strategy is do.call(rbind, by(dfrm, grp, FUN)).