[R] Mean(s) from values in different row?

Hello: Is there a way to get a mean from values stored in different rows? The data looks like this: YEAR-1, JAN, FEB, ..., DEC YEAR-2, JAN, FEB, ..., DEC YEAR-3, JAN, FEB, ..., DEC What I want is the mean(s) for just the consecutive winter months: YEAR-1.DEC, YEAR-2.JAN, YEAR-2.FEB YE

[R] Rekeying value denoting NA

I'm trying to rekey values which denote there is no values, i.e., '-999' in a dataset which contains both '-999' and NA entries. When I try the following command I get the following error: > data.frame[data.frame$MAR <= -9,"MAR"] <- NA "missing values are not allowed in subscripted assignment