merge data.frame but keep only unique columns?

Question

Let\'s say I want to merge two data.frames but some of the columns are redundant (the same). How would I merge those data.frames but drop the redundant columns?

Answer 1

You could include the column same in your by argument. The default is by=intersect(names(x), names(y)). Try merge(X1, X2) (it is the same as merge(X1, X2, by=c("id", "same"))):

 merge(X1,  X2)
 #  id same different1 different2
 #1  a    1          4          9
 #2  b    2          5          7
 #3  c    3          6          8

Answer 2

Just subset via indexing in the merge statement. There are many ways to subset i.e. name, position. There is even a subset function but the [] notation works well for almost all cases

merge(X1[,c("id","same","different1")], X2[,c("id","different2")], by="id", all = TRUE, sort = FALSE)

As shown in other examples you could put it into the by statement but this will become an issue after you exit the realm of one-to-one merges and enter one-to-many or many-to-many merges.