# Re: [R] cumulative sum of within levels of a dataframe

From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>
Date: Fri, 27 Jun 2008 22:09:35 +0100

On Fri, 2008-06-27 at 16:52 -0400, 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\$willdo <- unlist(tapply(df\$x, df\$fac, cumsum))
> df\$ideal <- df\$willdo - df\$x
> df

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

```

HTH G

>
> > 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.
>
> 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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Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
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Received on Fri 27 Jun 2008 - 21:12:14 GMT

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