Below is how I am currently doing this. Is there a more efficient way to do this?
The scenario is that I have two dataframes of different sizes. I need to update one binary factor variable in one of those dataframes by matching on two variables. If there is no match keep as is otherwise update. Also the variable being update, TT in this case should remain a binary factor variable (levels='HC','TER')

HTDF2<-merge(H_DF,T_DF,by=c("FY","ID"),all.x=T)
HTDF2$TT<-factor(ifelse(is.na(HTDF2$TT.y),HTDF2$TT.x,HTDF2$TT.y),labels=c("HC","TER"))
HTDF2<-HTDF2[,-(3:4)]


# REPRODUCIBLE EXAMPLE DATA FOR ABOVE..
> dput(H_DF)
structure(list(FY = structure(c(1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L,
5L), .Label = c("FY09", "FY10", "FY11", "FY12", "FY13"), class = "factor"),
    ID = c(1, 1, 1, 1, 2, 2, 2, 2, 2), TT = structure(c(1L, 1L,
    1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("HC", "TER"), class = "factor")), .Names = c("FY",
"ID", "TT"), class = "data.frame", row.names = c(1L, 2L, 3L,
4L, 6L, 7L, 9L, 10L, 11L))
> dput(T_DF)
structure(list(FY = structure(c(4L, 2L, 5L), .Label = c("FY09",
"FY10", "FY11", "FY12", "FY13"), class = "factor"), ID = c(1,
2, 2), TT = structure(c(2L, 2L, 2L), .Label = c("HC", "TER"), class = "factor")), .Names = c("FY",
"ID", "TT"), row.names = c(5L, 8L, 12L), class = "data.frame")

Dan Lopez
LLNL, HRIM - Workforce Analytics & Metrics

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