st: Creating repeated index

[TA Stat <tastat@gmail.com>]

Dear Stata Users

I am trying to create an index/running number based upon same value of
one variable, i.e., visit date.  Different visit date will have
different index/running number.  I can do it by finding duplicates and
drop them, then create an index and save as a new file, then merge a
new file with the original one.  However, I want to know whether there
is a short cut to do this.  My data will looks like this.

StudentID     Visit Date     Item       Index
1                  1/1/2010        A            1
1                  1/1/2010        B            1
1                  2/4/2010        A            2
1                  2/4/2010        B            2
1                  2/4/2010        C            2
2                  4/1/2010        A            1
2                  4/1/2010        B            1
2                  4/1/2010        C            1
2                  3/3/2010        A            2
2                  3/3/2010        E            2

Thanks
Pete
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