Question

I have a dataframe with three variables: ID, group, and nominated_ID.
I want to know the group that nominated_ID belongs in.

I’m imagining that for each case, we take nominated_ID, find the case where it is equal to ID, and then set the nominated_Group variable in the original case equal to the group variable in the matched case. (If there is no match, set it to NA)

I wouldn’t be surprised if this can be done without a loop, so I’m open-minded about the solution. Thanks so much for your help. Know that I did try to look for similar questions before posting.

Answer 1

You can achieve this in one step without the use of cbind by directly allocating results to a column in your data.frame:

df$nominated_group <- with(df, group[match(nominated_ID, ID)])
df
  ID group nominated_ID nominated_group
1  9   Odd            9             Odd
2  5   Odd            8            <NA>
3  2  Even            4            Even
4  4  Even            9             Odd
5  3   Odd            2            Even

I used with as a convenient way of referring to the columns of df without having to repeatedly write df$.