[R] Merge dataframes with no shared rows, some shared and som e unshared columns

[Liaw, Andy]

> first <- data.frame(a=1:3, b=4:6)
> second <- data.frame(b=7:9, c=10:12)
> third <- merge(first, second, by="b", all=TRUE)
> third
  b  a  c
1 4  1 NA
2 5  2 NA
3 6  3 NA
4 7 NA 10
5 8 NA 11
6 9 NA 12

It's easy to replace the Nas with whatever value you want.

No, merge() does not work with more than two data sets at a time.

Andy

From: mtb954 at gmail.com
> 
> Dear R-users,
> 
> I have two dataframes FIRST and SECOND that do not share any rows
> (i.e., there is no unique identifier linking the rows in the two
> dataframes, the rows are independent).
> 
> The dataframes have three variables (in columns) in common, but each
> dataframe also has some variables not shared by the other.
> 
> I wish to merge FIRST and SECOND into a THIRD dataframe, stacking the
> shared columns and including the unshared columns from FIRST and
> SECOND as columns in THIRD. Also, I wish rows in THIRD that do not
> have a value in a particular column (because they came from a
> dataframe without that column) to acquire the value "0" (zero).
> 
> Finally, it it possible to merge >2 dataframes using the 
> merge function?
> 
> Any ideas? Thanks, Mark
> 
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