Unexpected behavior using -which() in R when the search term is not found

Question

I have been using the R which function to remove rows from a data frame. I recently discovered that if the search term is NOT in the data.frame, the result is an e

Answer 1

That is a well-known pitfall. When nothing matches the logical test the which-function returns numeric(0) and then "[" returns nothing instead of returning everything which would be expected. You can use:

 LETTERS[ ! LETTERS == "1" ]
 LETTERS[ ! LETTERS %in% "1" ]

There is another gotcha to be aware of and is the one that makes me choose to use which(). When using logical indexing an NA value used inside "[" will return a row. I generally do not want that so I use DFRM[ which(logical) ] although this seems to bother some people who say is is not needed. I just think they are working with small datasets and infrequently encounter the annoyance of seeing tens of thousands of NA-induced useless lines of output on their console. I never use the negated which version though.