Looping grepl() through data.table (R)

Question

I have a dataset stored as a data.table DT that looks like this:

print(DT)
   category            industry
1: administration      admin
2: nurse         


        

Answer 1

Data.table is good at grouped operations; I think that's how it can help, assuming you have many rows with the same industry:

DT[ DT[, .I[grep(industry, category)], by = industry]$V1 ]

This uses the current idiom for subsetting by group, thanks to @eddi .

---

Comments. These might help further:

• If you have many rows with the same industry-category combo, try by=.(industry,category).

• Try something else in the place of grep (like the options in Ken and Richard's answers).