# Python: Validate Credit Card Numbers Using Floor Division (//) and Modulus (%) Operators (Ep. 3)

In this video, we’ll learn how to use While loops and the Floor Division (//) and Moduls (%) operators to solve the following problem. Make sure you watch Episode 1 if you need further explanations for the problem below.

☑️ The last digit of a credit card number is the check digit, which protects against transcription errors such as an error in a single digit or switching two digits. The following method is used to verify actual credit card numbers but, for simplicity, we will describe it for numbers with 8 digits instead of 16:

☑️ Starting from the rightmost digit, form the sum of every other digit. For example, if the credit card number is 4358 9795, then you form the sum 5 + 7 + 8 + 3 = 23.

☑️ Double each of the digits that were not included in the preceding step. Add all digits of the resulting numbers. For example, with the number given above, doubling the digits, starting with the next-to-last one, yields 18 18 10 8. Adding all digits in these values yields 1 + 8 + 1 + 8 + 1 + 0 + 8 = 27. Add the sums of the two preceding steps. If the last digit of the result is 0, the number is valid. In our case, 23 + 27 = 50, so the number is valid.

☑️ Some sample valid numbers to try: 41235680, 12345674, 98765431, 11224458, 45645645

☑️ Write a program that implements this algorithm. The user should supply an 8-digit number, and you should print out whether the number is valid or invalid.