Setting a dummy if names appear in the previous year!

[River Huang]

Dear All, Suppose that I have data

* Example generated by -dataex-. To install: ssc install dataex
clear
input float(x year) str8 city float change
1 1999 "A" 0
1 1999 "B" 0
1 2000 "C" 0
1 2000 "A" 1
1 2001 "D" 0
1 2001 "E" 0
2 1999 "X" 0
2 1999 "W" 0
2 2000 "W" 1
2 2000 "X" 1
2 2001 "Z" 0
2 2001 "X" 1
end

For each `x', I 'd like to define a dummy (`change'). If the `city' names (in the current year) appeared in the previous year (one year before only), then `change'=1, 0 otherwise. Thanks.

[William Lisowski]

For this data, at least, the following code seems to do what you want.

. bysort x city (year): generate wanted = year[_n-1] == year-1

. sort x year city

. list, sepby(x year) noobs

  +-----------------------------------+
  | x   year   city   change   wanted |
  |-----------------------------------|
  | 1   1999      A        0        0 |
  | 1   1999      B        0        0 |
  |-----------------------------------|
  | 1   2000      A        1        1 |
  | 1   2000      C        0        0 |
  |-----------------------------------|
  | 1   2001      D        0        0 |
  | 1   2001      E        0        0 |
  |-----------------------------------|
  | 2   1999      W        0        0 |
  | 2   1999      X        0        0 |
  |-----------------------------------|
  | 2   2000      W        1        1 |
  | 2   2000      X        1        1 |
  |-----------------------------------|
  | 2   2001      X        1        1 |
  | 2   2001      Z        0        0 |
  +-----------------------------------+

[River Huang]

Dear William, Many thanks for this wonderful solution.