First and Last Occurrences in Stata - stata

Question

I have a panel dataset from 2006 to 2012. I generated a new variable entry that takes a value of 1 for the firm that entered to a country. For instance if a firm has missing value (.) for its sales at time (t) it takes a value of 0 and at (t+1) if it enters to a country in other words has a value for its sales it takes a value of 1. The successful command that I used for this is as follows:
egen firm_id=group(firm country)
by firm_id (year), sort: gen byte entry = ///
sum(inrange(sales, 0,.)) == 1 & sum(inrange(sales[_n - 1],0,.)) == 0
Since my data start from 2006 I excluded the observations for this year with the command:
bysort firm (year) : replace entry = 0 if year == 2006
However what I want is instead of having 0 values,
to have missing values for the subsequent years after its entry (e.g. at t+2 or t+3).
The same I applied for the exit but I changed the sort order of year:
gen nyear = -year
by firm_id (nyear), sort: gen byte exit = ///
sum(inrange(sales, 0,.)) == 1 & sum(inrange(sales[_n - 1],0,.)) == 0
since the last observation year in my data is 2012 I excluded those observations:
bysort firm (year) : replace exit = 0 if year == 2012
Again here what I want is instead of having 0 values,
to have missing values for the subsequent years after its exit (e.g. at t+2 or t+3).

Answer 1

As I understand it the variable sales is missing when are none and positive otherwise.
You want indicators for a year being the first and last years of sales for a firm in a country.
I think this gets you most of the way. First, we need example data!
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(firm_id year sales)
1 2006 .
1 2007 .
1 2008 42
1 2009 42
1 2010 42
1 2011 .
1 2012 .
2 2006 .
2 2007 666
2 2008 666
2 2009 .
2 2010 .
2 2011 .
2 2012 .
end
The first and last dates are the minimum and maximum dates, conditional on there being sales.
egen first = min(cond(sales < ., year, .)), by(firm_id)
egen last = max(cond(sales < ., year, .)), by(firm_id)
For discussion of technique, see section 9 of this paper. Then (1, .) indicators follow directly
generate isfirst = cond(year == first, 1, .)
generate islast = cond(year == last, 1, .)
list, sepby(firm_id)
+----------------------------------------------------------+
| firm_id year sales first last isfirst islast |
|----------------------------------------------------------|
1. | 1 2006 . 2008 2010 . . |
2. | 1 2007 . 2008 2010 . . |
3. | 1 2008 42 2008 2010 1 . |
4. | 1 2009 42 2008 2010 . . |
5. | 1 2010 42 2008 2010 . 1 |
6. | 1 2011 . 2008 2010 . . |
7. | 1 2012 . 2008 2010 . . |
|----------------------------------------------------------|
8. | 2 2006 . 2007 2008 . . |
9. | 2 2007 666 2007 2008 1 . |
10. | 2 2008 666 2007 2008 . 1 |
11. | 2 2009 . 2007 2008 . . |
12. | 2 2010 . 2007 2008 . . |
13. | 2 2011 . 2007 2008 . . |
14. | 2 2012 . 2007 2008 . . |
+----------------------------------------------------------+
I have done anything different for 2006 or 2012. You could just build special rules into the cond() syntax.