r - more dynamic melting with data.table

Question

I am looking for the most efficient form to transform

  ARTNR           FILGRP
1     1             9827
2     2             9348
3     3 9335, 9827, 9339

into this

  ARTNR      FILGRP
1     1      9827
2     2      9348
3     3      9335
4     3      9827
5     3      9339

I tried the following code and it works, but it is not elegant and has some shortcomings. :

setDT(artnrs)  
artnrs[, c("P1", "P2", "P3") := tstrsplit(FILGRP, ",", fixed=TRUE)] # 1)
artnrs <- melt(artnrs, c("ARTNR"), measure = patterns("^P")) # 2)
artnrs[,variable:=NULL] # 3)
artnrs <- na.omit(artnrs, cols="value") # 4)
names(artnrs)[2] <- "FILGRP" # 5)

• ad 1) splits the last column in three new ones. How can I make this dynamic and make it fit for five or ten?
• ad 2-5) rather clumpsy operations, could I chain this better?

It is based on data.table but performance is not that critical so an easy to understand tidyverse solution would be ok. But the fewer packages, the better.

Thanks!

dput output;

structure(list(ARTNR = c(1, 2, 3), FILGRP = c("9827", "9348", "9335, 9827, 9339")), 
row.names = c(NA, -3L), class = "data.frame")

question from:https://stackoverflow.com/questions/65943165/more-dynamic-melting-with-data-table

Answer 1

df <- structure(list(ARTNR = c(1, 2, 3), FILGRP = c("9827", "9348", "9335, 9827, 9339")), 
          row.names = c(NA, -3L), class = "data.frame")

df2 <- strsplit(df$FILGRP, split = ",")
df2 <- data.frame(ARTNR = rep(df$ARTNR, sapply(df2, length)), FILGRP = unlist(df2))