This one should be easy but it's giving me a hard time mostly because tapply
puts the results in a list. I want to calculate the cumulative sum of a
variable in a dataframe, but with the accumulation only within each level of
a factor. For a very simple example, take:
> df <-
data.frame(x=c(rep(1,5),rep(2,5),rep(3,5)),fac=gl(3,5,labels=letters[1:3]))
> df
x fac
1 1 a
2 1 a
3 1 a
4 1 a
5 1 a
6 2 b
7 2 b
8 2 b
9 2 b
10 2 b
11 3 c
12 3 c
13 3 c
14 3 c
15 3 c
I'd like to create another column in the dataframe so it looks like this,
and make sure that the cumulative sums still match the right levels of the
factor. I've included a "willdo" column that's just a cumulative sum, and
an "ideal" column that's the cumulative sum minus the current value - the
column headings are self explanatory.
> answer
x fac willdo ideal
1 1 a 1 0
2 1 a 2 1
3 1 a 3 2
4 1 a 4 3
5 1 a 5 4
6 2 b 2 0
7 2 b 4 2
8 2 b 6 4
9 2 b 8 6
10 2 b 10 8
11 3 c 3 0
12 3 c 6 3
13 3 c 9 6
14 3 c 12 9
15 3 c 15 12
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