Some times the best loop is not to loop at all. The Replace Field Contents tool can be used to assign the same value, a serial number or a calculated value to the same field of every record in your found set.
A looping primer:
enclose the body of your loop between the script steps Loop and End Loop.
Use the script step Exit Loop If  to check and see if conditions for exiting a loop have occurred and if so exit the loop. (Execution continues with the next step after the End Loop step.)
When you are looping through a found set of records and want to exit the loop after processing the last record in your found set, use Go To Record [next; exit after last].
Thanks a lot.
:smileywink:In this order:
1- Had a shower
2- Read your answer
4- Ready to implement