Simple question on finding duplicates

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Jeff
Sent: Wednesday, July 25, 2012 3:06 PM
To: r-help at r-project.org
Subject: [R] Simple question on finding duplicates


   I'm  trying  to find duplicate values in a column of a data frame.
For
   example, dataframe (a) below has two 3's. I would like to mark each
value of
   each row as either not being a duplicate of the one before (0), or
as a
   duplicate (1) - for example, as in dataframe (b). In SPSS, I would
simply
   compare each value to it's "lagged" value, but I can't figure out
how to do
   this with R.
   Can someone point me in the right direction?
   Thanks
   a <- data.frame( col1 = c(1,2,3,3,4))
   b <- data.frame( col1 = c(1,2,3,3,4), duplicate = c(0,0,0,1,0))