Select only the first row when merging data frames with multiple matches

Question

I have two data frames, \"data\" and \"scores\", and want to merge them on the \"id\" column:

data = data.frame(id = c(1,2,3,4,5),
                  state =          


        

Answer 1

Using data.table along with mult = "first" and nomatch = 0L:

require(data.table)
setDT(scores); setDT(data) # convert to data.tables by reference

scores[data, mult = "first", on = "id", nomatch=0L]
#    id score state
# 1:  1    66    KS
# 2:  2    86    MN
# 3:  3    76    AL

For each row on data's id column, the matching rows in scores' id column are found, and the first one alone is retained (because mult = "first"). If there are no matches, they're removed (because of nomatch = 0L).