[R] How to select only certain rows when making a new dataframe?

[Marco Geraci]

I think ?merge is what you need. The help page of this
function is very....helpful

Marco

--- mtb954 at gmail.com wrote:

> Dear R-users,
> 
> I have two data frames. The "FIRST" data frame has
> 100 rows, the
> "SECOND" data frame has only 50 rows.
> 
> The data frames have different variables in columns
> ("VAR1," "VAR2,"
> etc) but they share a column called "ID" that
> contains a unique
> identifer linking the two data frames.
> 
> I would like to make a "THIRD" data frame containing
> just the rows of
> the "FIRST" data frame that match the rows (on "ID")
> in the "SECOND"
> data frame.
> 
> >THIRD=data.frame(FIRST$ID,FIRST$VAR1,FIRST$VAR2)
> 
> How can I modify this line to include in "THIRD"
> just the rows in
> "FIRST" that match the rows in "SECOND"? (i.e.,
> contain the same value
> in the shared "ID" column).
> 
> Thanks! Mark
> 
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