I have a "data frame" like this:

  Category Observed_value  A 100  A 130  A 140  B 90  C 80  D 120  D 130

I need to create an index column that show the number of the observation
for each category. I have three observations in the category A, one in
category B, one in category C and two in category D. The new matrix should
be like this:


  Category Observed_value Index  A 100 1  A 130 2  A 140 3  B 90 1  C 80 1
D 120 1  D 130 2

Please, could anyone help me with this problem?

Thank you.
-- 
Marcos Martins Santos
mmsantos.br@gmail.com
--
Skype: marcosmartinssantos

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