[R] cumulative sum of within levels of a dataframe

[Erik Iverson]

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
> 
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> 
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