I have a data set like the following:

subject visit x1
1 1 0.5
1 2 1.2
1 3 0.7
2 1 0.4
2 2 0.6
2 3 1.0
.....

where x1 is the interval between the two visits. Now I want to calculate the
cumulative intervals since the beinging, for example

 subject visit x1 cum
1 1 0.5 0.5
1 2 1.2 0.5+1.2
1 3 0.7 0.5+1.2+0.7
2 1 0.4 0.4
2 2 0.6 0.4+0.6
2 3 1.0 0.4+0.6+1.0
.....

is there an easy to generate the last veriable cum? The number of visits for
each subject may be different.

I also want to choose a subset which correspond to the last observation of
each subject. I am also wondering if there is a fast way to do so.

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