R data.table binary value for last row in group by condition

Question

I have data like this:

library(data.table)
id <- c(\"1232\",\"1232\",\"1232\",\"4211\",\"4211\",\"4211\")
conversion <- c(0,0,0,1,1,1)
DT <- data.ta         


        

Answer 1

Filter for the last row per group and set lastconv equal to conversion.

DT[DT[, .I[.N], by=id]$V1, lastconv := conversion]

Then replace NAs with 0

DT[is.na(lastconv), lastconv := 0L]

Result

DT
#     id conversion lastconv
#1: 1232          0        0
#2: 1232          0        0
#3: 1232          0        0
#4: 4211          1        0
#5: 4211          1        0
#6: 4211          1        1

---

If data.table v1.12.3 is installed we could also use the new function setnafill to replace NAs in the second step

DT[DT[, .I[.N], by=id]$V1, lastconv := conversion]
setnafill(DT, cols = "lastconv", fill = 0L)