[datatable-help] I have been agnozing over how to do a running cummulative sum over a particular date range

[Mike.Gahan]

But what if the dates are irregularly spaced?

#Build some sample data 
set.seed(12345)
data <- data.table(Date=seq(1,60,by=3),Value=rpois(20,10)) 

#Build reference table.  This is where we keep the list of Dates and Values
that will be referenced for 
#each individual data 
Ref <- data[,list(Compare_Value=list(I(Value)),Compare_Date=list(I(Date)))] 

#Use lapply to get last seven days of value by id 
data[,Roll.Val := lapply(Date, function(x) { 
                  d <- as.numeric(Ref$Compare_Date[[1]] - x) 
                  sum((d <= 0 & d >= -7)*Ref$Compare_Value[[1]])})] 

head(data,10) 

    Date Value Roll.Val
 1:    1    12       12
 2:    4     9       21
 3:    7    10       31
 4:   10    10       29
 5:   13    14       34
 6:   16    13       37
 7:   19     7       34
 8:   22    12       32
 9:   25    12       31
10:   28    16       40



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