Using grep to subset rows from a data.table, comparing row content

Question

DT <- data.table(num=c(\"20031111\",\"1112003\",\"23423\",\"2222004\"),y=c(\"2003\",\"2003\",\"2003\",\"2004\"))

> DT
    num    y
1: 20031111 2003
2:  111200         


        

Answer 1

You could do this

DT[, x := grep(y, num, value = TRUE, fixed = TRUE), by = .(num, y)]

#> DT
#        num    y        x
#1: 20031111 2003 20031111
#2:  1112003 2003  1112003
#3:    23423 2003       NA
#4:  2222004 2004  2222004