Michael MacAskill - 4 months ago 21

R Question

When examining datasets from longitudinal studies, I commonly get results like this from a

`dplyr`

`df = data.frame(n_sessions=c(1,2,3,4,5), n_people=c(59,89,30,23,4))`

i.e. a count of how many participants have completed a certain number of assessments at this point in time.

Although it is useful to know how many people have completed

`n_total`

`n_people`

`n_sessions n_people n_total cumsum`

1 59 205 59

2 89 146 148

3 30 57 178

4 23 27 201

5 4 4 205

Generating the cumulative sum is simple:

`mutate(df, cumsum = cumsum(n_people))`

What would be an expression for generating a "forwards cumulative sum" that could be incorporated in a

`dplyr`

`cumsum`

`n_people`

`n_sessions`

Answer

You can take a cumulative sum of the reversed vector, then reverse that result. The built-in rev function is helpful here:

```
mutate(df, rev_cumsum = rev(cumsum(rev(n_people))))
```

For example, on your data this returns:

```
n_sessions n_people rev_cumsum
1 1 59 205
2 2 89 146
3 3 30 57
4 4 23 27
5 5 4 4
```