Count distinct by group- moving window

Question

Let\'s say I have a dataset contain visits in a hospital. My goal is to generate a variable that counts the number of unique patients the visitor has seen before at the date

Answer 1

You can do:

with(df, ave(patient, visitor, FUN = function(x) cumsum(!duplicated(x))))

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Essentially, it is a cumulative sum of non-duplicated values per group.

And you can also do the same with dplyr:

df %>%
 group_by(visitor) %>%
 mutate(res = cumsum(!duplicated(patient)))