Filling field with automatic numbering specific by unique value of another field using QGIS expression

Question

I have a table with two fields: "NUMBER" (integer) and "TOWN" (text). I would like to find an expression that fills the "NUMBER" field with an automatic number specific for each value of "TOWN".

Example:

NUMBER | TOWN
----------------------
1      | STRASBOURG
2      | STRASBOURG
3      | STRASBOURG
1      | PARIS
2      | PARIS
1      | TOULOUSE
2      | TOULOUSE
3      | TOULOUSE
4      | TOULOUSE

How can I do that with a QGIS expression?

Answer 1

Finally, I found the right expression to do that:

"TOWN" || (@row_number - minimum(@row_number, "TOWN") + 1)