CREDIT CARD CHECKING PROGRAM

This program is to check a credit card number weather it is valid or not .A valid credit card is a 16 digit number would satisfy the following rule.

• from rightmost digit -1 every digit is multiplied by 2.

• and digits which is multiplied by 2 should not be greater than 10.

• and sum of all the digits will be equal to 38.

• and sum of other digits which is not multiplied by 2 will be equal to 42.

• and adding sum1 and sum2 will give 80 which is divisible by 10 

then this will be a valid credit card number.
The following dummy credit card number will show all the above given points:

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 the numbers greater than 10
8 3 2 6 1 5 9 4
Add them all up to get 38.
Add all other digits to get 42 ,the digits which is not multiplied by 2:
   5      7                2      4      6      8      1      9
sum of 32 and 42 is 80 is divisible by 10, the credit card is valid
here it is given in the program:

#include<stdio.h>

#include<conio.h>

void main()

{

int a[16]={4,5,6,7,1,2,3,4,5,6,7,8,9,1,2,9},i,sum=0,sum2=0;

int s;

clrscr();

for(i=14;i>=0;i=i-2)

{

a[i]=a[i]*2;

if(a[i]>=10)

{

a[i]=a[i]-9;

}

sum+=a[i];

}

for(i=15;i>0;i=i-2)

{

sum2+=a[i];

}

s=sum+sum2;

if(s==80&&s%10==0)

printf("The credit card is valid");

else

printf("The credit card is not valid");

          

       getch();

}