[R] Retain only those records from a dataframe that exist in another dataframe

[Peter Dalgaard]

"Mark Na" <mtb954 at gmail.com> writes:

> Dear R community,
> 
> I have two dataframes "first" and "second" which share a unique identifier.
> 
> I wish to make a new dataframe "third" retaining only the rows in
> "first" which also occur in "second".
> 
> I have tried using merge but can't seem to figure it out. Any ideas?

Doesn't sound like a merge problem. Will this do it?:

first[first$ID %in% second$ID,]

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