The program should ask the user for the digits (between 13 and 16) of his credit

Question

The program should

ask the user for the digits (between 13 and 16) of his credit card, and tell him if

his

(when the first number is 4 for Visa cards

5 for Master cards

37 for American Express cards

6 for Discover cards) is valid or not valid.

As a reminder, LUHN algorithm, take first, third, fifth, seventh,nineth, eleventh, thirteenth, fifteenth(when applies), digits. multiply it by 2 and IF the number obtained is bigger than 10 you ad the digits obtained (7 --> 7*2=14 --> 1+4=5)

You'll now get a new card number,
7175280148901 will become 5155480188902
5+1+5+5+4+8+0+1+8+8+9+0+2= 56

therefore 7175280148901 isn't valid and 7175280148901

Explanation / Answer

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

int main()

{

char d[20];

printf("Enter the credit card no: ");

scanf("%s",d);

int n=0;

int l= strlen(d);

int i=0;

int sum=0;

while(i<l)

{

n=d[i]-'0';

if(i%2 == 0)

{

n=n*2;

if(n>9) { n=n-9; }

}

sum=sum+n;

i++;

//if you want to print the new number, uncomment the following two printfs

//printf("%d",n);

}

//printf(" ");

if(sum%10 == 0 )

{ printf("The number is VALID "); }

else

{ printf("The number is INVALID "); }

return 0;

}

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