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

Sounds important with what you are tracking. Another option using data.table using non-equi join and then update by reference:

DT[, goal2 :=
    DT[.SD, on=.(visitor, visitdt<=visitdt), allow.cartesian=TRUE, 
        length(unique(patient)), by=.EACHI]$V1]

output:

    visitor    visitdt patient goal goal2
 1:  125469 2018-01-12   15200    1     1
 2:  125469 2018-01-19   15200    1     1
 3:  125469 2018-02-16   15200    1     1
 4:  125469 2018-02-23   52607    2     2
 5:  125469 2018-03-09   52607    2     2
 6:  125469 2018-03-16   52607    2     2
 7:  125469 2018-03-23   15200    2     2
 8:  125469 2018-03-29   15200    2     2
 9:  125469 2018-03-30   20589    3     3
10:  125469 2018-04-06   20589    3     3

data:

library(data.table)
DT <- fread("visitor visitdt patient goal
125469  1/12/2018   15200   1
125469  1/19/2018   15200   1
125469  2/16/2018   15200   1
125469  2/23/2018   52607   2
125469  3/9/2018    52607   2
125469  3/16/2018   52607   2
125469  3/23/2018   15200   2
125469  3/29/2018   15200   2
125469  3/30/2018   20589   3
125469  4/6/2018    20589   3")
DT[, visitdt := as.Date(visitdt, "%m/%d/%Y")]