how to get field names dynamically from an internal table

Hi,

I am having an internal table with fields

balance

a1

a2

a3

a4

a5

a6

amount

all these fields has value except balance.

i need to calculate balance for each record in this internal table.

on the selection screen user will give a1 to a4 or a3 to a6 or a2 to a5 etc as an input.

The logic for calculating balance is if user has given input as a2 to a5 then

balance = a2 + a3 + a4 + a5 + amount.

if user gives input as a1 to a4 then

balance = a1 + a2 + a3 + a4 + amount.

so dynamically i need to change the field names of the internal table depending upon the user input to calculate balance.

can anyone explain me how to get those field values dynamically from the internal table. Thanks...

Regards,

Rose.