Remove multiple columns from data.table

Question

What's the correct way to remove multiple columns from a data.table? I'm currently using the code below, but was getting unexpected behavior when I accidentally repeated one of the column names. I wasn't sure if this was a bug, or if I shouldn't be removing columns this way.

library(data.table) DT <- data.table(x = letters, y = letters, z = letters) DT[ ,c("x","y") := NULL] names(DT) [1] "z" 

The above works fine, but

DT <- data.table(x = letters, y = letters, z = letters) DT[ ,c("x","x") := NULL] names(DT) [1] "z" 

Answer 1

This looks like a solid, reproducible bug. It's been filed as Bug #2791.

It appears that repeating the column attempts to delete the subsequent columns.
If no columns remain, then R crashes.

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UPDATE : Now fixed in v1.8.11. From NEWS :

Assigning to the same column twice in the same query is now an error rather than a crash in some circumstances; e.g., DT[,c("B","B"):=NULL] (delete by reference the same column twice). Thanks to Ricardo (#2751) and matt_k (#2791) for reporting. Tests added.