Removing specific rows from a dataframe

Question

I have a data frame e.g.:

sub   day 1      1 1      2 1      3 1      4 2      1 2      2 2      3 2      4 3      1 3      2 3      3 3      4 

and I would like to remove specific rows that can be identified by the combination of sub and day. For example say I wanted to remove rows where sub='1' and day='2' and sub=3 and day='4'. How could I do this? I realise that I could specify the row numbers, but this needs to be applied to a huge dataframe which would be tedious to go through and ID each row.

Answer 1

DF[ ! ( ( DF$sub ==1 & DF$day==2) | ( DF$sub ==3 & DF$day==4) ) , ]   # note the ! (negation) 

Or if sub is a factor as suggested by your use of quotes:

DF[ ! paste(sub,day,sep="_") %in% c("1_2", "3_4"), ] 

Could also use subset:

subset(DF,  ! paste(sub,day,sep="_") %in% c("1_2", "3_4") ) 

(And I endorse the use of which in Dirk's answer when using "[" even though some claim it is not needed.)