R - How to Remove Items in a Vector that Satisfy One of Several Conditions

Question

I have a large dataframe which contains a column of names, and given the nature of my data the names repeat. I also have a vector of a subset of those names that I need to eliminate from that dataframe. So I want to identify the row number for each instance that the name in the dataframe matches a name in the list of names to be dropped. Here is an example of what I'm trying to do...but I can't get the code to work. Thanks!

a=c("tom", "bill", "sue", "jim", "tom", "amy")
b=c(12,15,7,22,45,5)

ab=data.frame(a,b)

ab

drop=which(ab$a==c("tom", "sue")) #only identifies those matching "tom"

drop

ab2=ab[-drop,]

ab2

Answer 1

you're looking for %in%

drop=which(ab$a %in% c("tom", "sue"))

however, more succinctly:

ab[!ab$a %in% c('tom', 'sue'),]

Answer 2

You should have a look at the package sqldf. You may perform SQL Selects on R Data Frames.