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, the card number is valid; otherwise, it is invalid. For example, the number 4388576018402626 is invalid, but the number 4388576018410707 is valid.

int luhn(const char* cc)
{
const int m[] = {0,2,4,6,8,1,3,5,7,9}; // mapping for rule 3
int i, odd = 1, sum = 0;

``````for (i = strlen(cc); i--; odd = !odd) {
int digit = cc[i] - '0';
sum += odd ? digit : m[digit];
}

return sum % 10 == 0;
``````

}

so this is the function to applys the luhn's algorithm, but i'm not sure how to input the credit card number from the main function using scanf and connects it to this function

If you don't know C's most basic operations, one questions what you're doing working with credit card validation algorithms or code.

Hmmmmm.

well i just started programming one and a half month ago, but my professor is teaching at a really fast pace, and i couldnt cope with it.
Most of my classmates have problems too, so i guess we're really in a big trouble.

A credit card number must have between 13 and 16 digits

The current max is already on 19.

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