Stata interaction variable base year

Question

My data is in the following format:

clear all
input year  ID  var1    var2
2000    1   100 .
2001    1   200 0.2
2002    1   300 0.3
2000    2   500 .
2001    2   300 0.4
2002    2   400 0.1
end

I would like to create an interaction variable between var1 and var2 where var1 always takes the value of the first year where I have data (in this case year 2000 as the base year).

year	ID	var1	var2	var1xvar2
2000	1	100	.	.
2001	1	200	0.2	20
2002	1	300	0.3	30
2000	2	500	.	.
2001	2	300	0.4	200
2002	2	400	0.1	50

How can I achieve this?

Answer 1

The following works for me:

bysort ID (year): gen wanted = var1[1] * var2

list, sepby(ID)

     +----------------------------------+
     | year   ID   var1   var2   wanted |
     |----------------------------------|
  1. | 2000    1    100      .        . |
  2. | 2001    1    200     .2       20 |
  3. | 2002    1    300     .3       30 |
     |----------------------------------|
  4. | 2000    2    500      .        . |
  5. | 2001    2    300     .4      200 |
  6. | 2002    2    400     .1       50 |
     +----------------------------------+

The [1] refers to the first observation in each ID panel, which in this case is the lowest value of year because the dataset is sorted by ID and year.