[R] combine two data-frames into one

[Suyan Tian]

Hi Everyone:

I need some help on combining two datasets into one.  Since for those  
two datasets, there are different variables. Like

In dataset 1, I have

pt  x1 x2
1  12  24
2   20 17

In dataset 2, I have
pt  x2   x3
3   19  33
4   20  16

The final dataset I want should look as
pt  x1 x2 x3
1   12 24 NA
2   20 17 NA
3   NA 19 33
4  NA  20 16

I used setdiff and rbind together and got what I wanted, but I am  
thinking there must be a simpler way to do this. Maybe just one  
command or something. Please help me out.

Thanks so many,

Suyan