Creating a dummy variable based on three other variables

[Alexandre Loures]

I have a variable year (2003 and 2013), an export (dummy 0 and 1) and an idcode. I would like to create a dummy as follows: if for an idcode in 2003 export is 0 but in 2013 it is 1 then the dummy would be 1. For example:

year    export       idcode
2003       1         6000146
2003       0         3047347
2013       1         6000146
2013       1         3047347

Then, the new base will be:

year     export        idcode        dummy
2003        1          6000146         0
2003        0          3047347         0
2013        1          6000146         0
2013        1          3047347         1

Can anybody help me? Thanks in advance for your attention!

[Nick Cox]

clear 
input year    export       idcode
2003       1         6000146
2003       0         3047347
2013       1         6000146
2013       1         3047347
end 

egen wanted = total((year == 2003 & export == 0) | (year == 2013 & export == 1)), by(id)

sort idcode year

list, sepby(idcode) 

     +----------------------------------+
     | year   export    idcode   wanted |
     |----------------------------------|
  1. | 2003        0   3047347        2 |
  2. | 2013        1   3047347        2 |
     |----------------------------------|
  3. | 2003        1   6000146        1 |
  4. | 2013        1   6000146        1 |
     +----------------------------------+

replace wanted = wanted == 2 

list, sepby(idcode) 

     +----------------------------------+
     | year   export    idcode   wanted |
     |----------------------------------|
  1. | 2003        0   3047347        1 |
  2. | 2013        1   3047347        1 |
     |----------------------------------|
  3. | 2003        1   6000146        0 |
  4. | 2013        1   6000146        0 |
     +----------------------------------+