Generate a variable based on id number and its chosen indicator

[Chen Samulsion]

Dear Stata users,

I encounter this question when using one-to-one matching. The variable id is number of observations, the variable chosen stores the observation number of the matched observation. I want to generate an indicator variable (i.e. matched) that equals 1 if id number can be found in corresponding variable chosen, and equals 0 if not. For example, matched in line three will take value 1 because you can find number 3 in variable chosen, line six also takes value 1 because you can find number 6 in variable chosen. I can't find a solution, hope someone could help me. Thank you!

* Example generated by -dataex-. To install: ssc install dataex
clear
input float(id chosen matched)
 1  . 1
 2  . .
 3  . 1
 4  . .
 5  . .
 6  . 1
 7  . 1
 8  . .
 9  . 1
10  . .
11  . 1
12  . .
13  . .
14  . 1
15  . .
16  . .
17  . 1
18  . .
19  1 .
20  3 .
21  6 1
22  7 1
23  9 .
24 11 1
25 14 .
26 17 1
27 21 .
28 22 .
29 24 .
30 26 .
end
label var id "observations ID"
label var chosen "chosen observations (value==id)"
label var matched "indicator that observation was choosed"

[Andrew Musau]

If you search the forum, there are many examples of this kind of problem. Here is a solution first suggested by Romalpa Akzo

expand 2, gen(set)
replace id=chosen if set
bysort id: egen wanted = max(id==id[_n+1]) if !missing(id)
drop if set

Result:

. list id chosen matched wanted

     +--------------------------------+
     | id   chosen   matched   wanted |
     |--------------------------------|
  1. |  1        .         1        1 |
  2. |  2        .         .        0 |
  3. |  3        .         1        1 |
  4. |  4        .         .        0 |
  5. |  5        .         .        0 |
     |--------------------------------|
  6. |  6        .         1        1 |
  7. |  7        .         1        1 |
  8. |  8        .         .        0 |
  9. |  9        .         1        1 |
 10. | 10        .         .        0 |
     |--------------------------------|
 11. | 11        .         1        1 |
 12. | 12        .         .        0 |
 13. | 13        .         .        0 |
 14. | 14        .         1        1 |
 15. | 15        .         .        0 |
     |--------------------------------|
 16. | 16        .         .        0 |
 17. | 17        .         1        1 |
 18. | 18        .         .        0 |
 19. | 19        1         .        0 |
 20. | 20        3         .        0 |
     |--------------------------------|
 21. | 21        6         1        1 |
 22. | 22        7         1        1 |
 23. | 23        9         .        0 |
 24. | 24       11         1        1 |
 25. | 25       14         .        0 |
     |--------------------------------|
 26. | 26       17         1        1 |
 27. | 27       21         .        0 |
 28. | 28       22         .        0 |
 29. | 29       24         .        0 |
 30. | 30       26         .        0 |
     +--------------------------------+

[Chen Samulsion]

Thank you Andrew Musau, a nice solution!