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

Here is a base R method using aggregate and head:

merge(data, aggregate(score ~ id, data=scores, head, 1), by="id") 

The aggregate function breaks up the scores dataframe by id, then head is applied to get the first observation from each id. Since aggregate returns a data.frame, this is directly merged onto the data.frame data.

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Probably more efficient is to subset the scores data.frame using duplicated which will achieve the same result as aggregate, but will reduce the computational overhead.

merge(data, scores[!duplicated(scores$id),], by="id")