[R] Combinations of values in two columns

[Thomas]

I have data that looks like this:

Friend1, Friend2
A, B
A, C
B, A
C, D

And I'd like to generate some more rows and another column. In the new  
column I'd like to add a 1 beside all the existing rows. That bit's  
easy enough.

Then I'd like to add rows for all the possible directed combinations  
of rows not included in the existing data. So for the above I think  
that would be:

A, D
D, A
B, C
C, B
B, D
C, A
D, B
D, C

and then put a 0 in the column beside these.

Can anyone suggest how to do this?

I'm using R version 2.15.3.

Thank you,

Thomas Chesney
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