Loop form datasource

Good morning ,

As i have a requirement when i open a form and click at any point i mean any record

and i will click one button then there is one customized field on form which should generate number based on input given at the time of clicking that button.

Ex - i opened form

i selected 5 th record and i clicked button then dialog opens which ask me start from i will write 1

then numbers should generate from 5.

i tried looping through this

for(fdpaymentjournal = paymentJournalfds.getNext() ; fdpaymentjournal ; fdpaymentjournal = paymentJournalfds.getNext() )
{

}

i tried even get first() and get selection but no luck

Please help me if i select any record that form datasource should loop in my class

please show some light on this.

Regards.

Have a great day.