I have a very simple issue, and I read some help material but can't quite solve it.
I want to create some new IDs identifiers combining the session number and the session IDs of my experiment. However, I want these new IDs to have the same number of digits. So the result should be as follows:

session session_ID new_ID
1 1 101
1 2 102
...
1 10 110
1 11 111
...
2 1 201
2 2 202
...
2 10 210
2 11 211
...

If I used Excel, the code would be new_ID = concatenate(session; if(length(session_ID)=1;"0";"");session_ID)

How can this be done in Stata?
Thanks in advance for your help!
gianluca