Best approach to update the cdt in expression rule?

Hello All,

I've a CDT {{categoey:"a",serialNumber:null},{category:"a",serialNumber:null},{category:"a",serialNumber:null},{categoey:"b",serialNumber:null},{category:"b",serialNumber:null},{category:"b",serialNumber:null}}

And need to generate the serial number for each category, and my output cdt should look like

{{categoey:"a",serialNumber:1},{category:"a",serialNumber:2},{category:"a",serialNumber:3},{categoey:"b",serialNumber:1},{category:"b",serialNumber:2},{category:"b",serialNumber:3}}

I'm using 2 for loops and updatecdt function to get the expacted output.

foreach (loop {distinct categories})

 foreach(records for the category)

  updatecdt(array,serialnumber,fv!index)

Would like to check if there is any better approach to achieve the same.

Thank You!!