this program validates a credit card entered by the user
a credit card number is usually a 16-digit number.A valid credit card would satisfy a rule explained below::
start with the rightmost 2nd-digit and multiply it with 2 and skip another digit then mutiply that digit with 2 and keep doing this ...
then subtract 9 from any number larger than 10 then add them all up to get a number (say X)
and add all the other digits left in the entered credit card number to get a number (say Y)
then add up X and Y
if the sum is divisible by 10 then the credit card number is valid
small example ::
4 5 6 7 1 2 3 4 5 6 7 8 9 1 2 9 (entered credit card number)
8 12 2 6 10 14 18 4 (first line)
then add up "first-line" to get 38
then add up all the other numbers to get 42
adding them we get 80 which is divisible by 10 hence the credit card number is valid ;-)