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

[mtb954@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