Suppose I have a long format for a longitudinal data

id time x
1 1 10
1 2 11
1 3 23
1 4 23
2 2 12
2 3 13
2 4 14
3 1 11
3 3 15
3 4 18
3 5 21
4 2 22
4 3 27
4 6 29

I want to select the x values for each ID when time is equal to 3. When that
observation is not observed, then I want to replace it with the obervation
at time equal to 4. otherwise just use NA.

How can I implement this with a quick command?

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