create unique ID for each group

I want to merge dat1 and dat2 based on "ID" in order

Have you tried merge(dat1, dat2) ?

If ID is the common column (and no others), then that should be all you 
need to join (see ?merge).  And then order if needed.

Chris

Chris Stubben

Los Alamos National Lab
Bioscience Division
MS M888
Los Alamos, NM 87545