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

[Chuck Cleland]

THIRD <- merge(FIRST, SECOND, by="ID", all.x=FALSE, all.y=TRUE)

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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-- 
Chuck Cleland, Ph.D.
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