How to merge two dataframes using multiple columns as key?

Question

Say I have the following dataframes:

DF1 <- data.frame(\"A\" = rep(c(\"A\",\"B\"), 18),
                  \"B\" = rep(c(\"C\",\"D\",\"E\"), 12),
                  


        

Answer 1

As Akrun mentioned in comments, your lookup table (DF2) needs to be reduced to just its unique A/B combinations. For your current dataframe, this isn't a problem, but you will need additional rules if there are multiple possible values for the same combination. From there, the solution is easy:

DF2.u <- unique(DF2)
DF3 <- merge(DF1, DF2.u, all = T)

Note that this will produce a new dataframe with an empty TEST column (all values NA), and a VAL column assigned from DF2. To do exactly what you wanted (replace TEST with VAL where possible), here is some slightly clunkier code:

DF1$TEST <- merge(DF1, DF2.u, all = T)$VAL

EDIT: in response to your question, you can boil down DF2 if necessary quite simple:

DF2$C <- c(1:12) #now unique() won't work
DF2.u <- unique(DF2[1:3])

 A B VAL
1 A C   1
2 A D   3