A Credit Card number is usually a 16-digit number. A valid Credit Card number would satisfy a rule explained below with the help of a dummy Credit Card number - 4567 1234 5678 9129. Start with the rightmost - 1 digit and multiply every other digit by 2.

4 5 6 7    1 2 3 4    5 6 7 8    9 1 2 9
 8  12        2  6       10  14       18  4

Then subtract 9 from any number larger than 10. Thus we get
8  3  2  6  1  5  9  4

Add them all up to get 38.
Add all other digits to get 42.

Sum of 38 and 42 is 80. Since 80 is divisible by 10, the Credit Card the number is valid.

Write a program that receives a Credit Card number and checks using the above rule whether the Credit Card number is valid.

#include<stdio.h>
#include<conio.h>
int main()
{
	char num[20];
	int i, sum = 0;
	printf("\nEnter the 16 digit credit card number : ");
	scanf("%s", num);

	for (i = 0; i <= 15; i++)//Traversing all numbers
	{
		num[i] -= 48;//converting each character in numeral
		if ((i % 2))//if the number is on right so it will directly get summed
			sum = sum + num[i];
		else//if number is on left, so it will first get doubled
		{
			num[i] *= 2;
			if (num[i] >= 10)//if number is greater then or equal to 10 so it will subtracted from 9
				num[i] -= 9;
			sum = sum + num[i];	//summing number
		}
	}
	if (!(sum % 10))//if sum is divisble by 10 so number is valid
		printf("\nNumber is valid.");
	else
		printf("\nNumber is not valid.");
	_getch();
	return 0;
}