How to create a group variable from ID variable?

[Waqas Qureshi]

ID	_n1	_n2	_n3	category
67	889	192	78
78
101
192
203	724	538	889
538
724
889

The data is provided. In front of ID 67, there are three ID variables given. I want to create the column "category" , where if an ID variable is found in the columns _n1, _n2 or _n3, the category variable should read as "1", if there is no ID variable in the _n1, _n2, or _n3 column, it should read "0". The ID variable which has 3 variables infront of it, should have a category 2.

It should look like this:

ID	_n1	_n2	_n3	category
67	889	192	78	2
78				1
101				0
192				1
203	724	538	889	2
538				1
724				1
889				1

I will appreciate your help.

[Andrew Musau]

* Example generated by -dataex-. For more info, type help dataex
clear
input int(id _n1 _n2 _n3) byte category
 67 889 192  78 2
 78   .   .   . 1
101   .   .   . 0
192   .   .   . 1
203 724 538 889 2
538   .   .   . 1
724   .   .   . 1
889   .   .   . 1
end

forval i=1/3{
    qui levelsof _n`i', local(levs`i') sep(|)
}
local levs `levs1'|`levs2'|`levs3'
gen wanted=cond(!missing(_n3), 2, cond(regexm(trim(string(id, "%9.0f")), "(`levs')"), 1, 0))

Res.:

. l, sep(0)

     +-------------------------------------------+
     |  id   _n1   _n2   _n3   category   wanted |
     |-------------------------------------------|
  1. |  67   889   192    78          2        2 |
  2. |  78     .     .     .          1        1 |
  3. | 101     .     .     .          0        0 |
  4. | 192     .     .     .          1        1 |
  5. | 203   724   538   889          2        2 |
  6. | 538     .     .     .          1        1 |
  7. | 724     .     .     .          1        1 |
  8. | 889     .     .     .          1        1 |
     +-------------------------------------------+