st: generate a code variable

["irodriguez" <irodriguez@utdt.edu>]

I have a desbalanced panel data with data of firms with many years of observations for each

example:

firm     year   benefit

A          1        10

A          2        19

A          3          8

B          1          9

B          2          7

C          2          2

C          3        12

C          4          6

C          5          6

I need to create a numeric variable for firms

Example

firm     year   benefit   codfirm

A          1        10         1

A          2        19         1

A          3          8         1

B          1          9         2

B          2          7         2

C          2          2         3

C          3        12         3

C          4          6         3

C          5          6         3

 

How can I do it?

Thanks