[R] count the cumulative for each subject

[Daniel Malter]

Hi,

subject=c(1,1,1,2,2,2)
visit=c(1,2,3,1,2,3)
x1=c(0.5,1.2,0.7,0.4,0.6,0.1)

cum=NULL #initialize
for(i in 1:length(subject)){
cum[i]=sum(x1[subject[i]==subject&visit[i]>=visit])
} 

I am sure there is a way with tapply or similar functions that is
computationally more efficient. But if your dataset is not very large, the
above function does the job very well.

Cheers,
Daniel


-------------------------
cuncta stricte discussurus
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-----Ursprüngliche Nachricht-----
Von: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Im
Auftrag von gallon li
Gesendet: Monday, November 24, 2008 2:22 AM
An: r-help
Betreff: [R] count the cumulative for each subject

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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