[R] cumulative sum of within levels of a dataframe
Erik Iverson
iverson at biostat.wisc.edu
Fri Jun 27 23:05:27 CEST 2008
Just use ?unlist
df$willdo <- unlist(tapply(df$x, df$fac, cumsum))
df$ideal <- df$willdo - df$x
Levi Waldron wrote:
> 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
>
> [[alternative HTML version deleted]]
>
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