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# Thread: Credit Card Problem (While and Switch will be used)

1. ## Credit Card Problem (While and Switch will be used)

Hey folks. I am a beginner. There is a problem which I struggle a lot. If you can help me, I will be happy.

Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start with:

4 for Visa cards

5 for Master cards

37 for American Express cards
6 for Discover cards

In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card numbers. The algorithm is useful to determine if a card number is entered correctly or if a credit card is scanned correctly by a scanner. Almost all credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check, which can be described as follows (for illustration, consider the card number 4388576018402626):

1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number.
2 * 2 = 4
2 * 2 = 4
4 * 2 = 8
1 * 2 = 2
6 * 2 = 12 (1 + 2 = 3)
5 * 2 = 10 (1 + 0 = 1)
8 * 2 = 16 (1 + 6 = 7)
4 * 2 = 8

2. Now add all single-digit numbers from Step 1.
4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 37

3. Add all digits in the odd places from right to left in the card number.
6 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 38

4. Sum the results from Step 2 and Step 3.
37 + 38 = 75

5. If the result from Step 4 is divisible by 10, and it is starting with the correct numbers, which are given at the beginning of the document, and the number of digits is between 13 and 16, then the card number is valid; otherwise, it is invalid.
For example, the number 4388576018402626 is invalid, but the number 4388576018410707 is valid.

Write a program that prompts the user with a menu to enter a credit card number as a long integer as first option, and Quit as the second option. Display whether the number is valid or invalid.

2. ## Re: Credit Card Problem (While and Switch will be used)

Please post what you have done (and please use the code tags), with specific questions about where the problem is or what you are having a hard time with - there is a lot posted above and it helps to break the problem down. We are not a code service so posting just a homework assignment doesn't do much good.

Edit: and please do not double post - your other post has been removed.

3. ## Re: Credit Card Problem (While and Switch will be used)

I haven't start coding yet. I am trying to understand what should i do. For example;

4388576018402626

for that number, I have to seperate every second digit from right to left and all digits in the odd places from right to left in the card number. I know i have to use "While" for it. Besides, I assume I should do something like that;

(assume there is a int num above..)

= num %10
/10

but I don't know the exact solution. Rest of that not so hard. Just switch and Scanner.

4. ## Re: Credit Card Problem (While and Switch will be used)

I'll give you the same hint as I did i another thread.
```int number = 81723;
System.out.println(number % 10);
System.out.println(number / 10);
System.out.println(5 % 2);
System.out.println(6 % 2);```

5. ## The Following User Says Thank You to Junky For This Useful Post:

odun (March 29th, 2011)