st: Identifying and recording the first occurrence of an event by actor given category

[Erik Aadland <erikaadland@hotmail.com>]

Dear Statalist.
I have an unbalanced panel dataset.
This is the structure:
actor_id    year    category_id    event    
1           2000    1              .
1           2000    2              1
1           2001    2              1
2           2003    3              .
2           2003    2              1
2           2004    2              .

I want to generate a variable -first_occurrence- that identifies and records for each actor_id the first time the actor experienced event = 1 if the category = e.g. 2. I would like this -first occurrence- variable to capture the value of -year- at the time of first event occurrence. Some actors never experience event = 1.
For instance, if I track first occurrence by category_id = 2, this is what I look for:
actor_id    year    category_id    event    first_occurrence   
1           2000    1              .        2000
1           2000    2              1        2000
1           2001    2              1        2000
2           2003    3              .        2003
2           2003    2              1        2003
2           2004    2              .        2003

Any input or suggestions on this problem would be greatly appreciated.
Kind regards,
Erik. 		 	   		  
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