# Credit - Why Does it Fail Passing check50?

When sending my code to Check50, all the Credit Card checks go through ok but one:

:( identifies 4062901840 as INVALID expected "INVALID\n", not "VISA\n"

But doing the steps: 1) Multiply every other digit by 2, starting with the number’s second-to-last digit, and then add those products' digits together.

Digits: 4 1 9 6 4 2x: 8 2 18 12 8 add: 8 2 9 3 8 = 30

2) Add the sum to the sum of the digits that weren’t multiplied by 2. Digits: 0 8 0 2 0 = 10 + 30 = 40

3) If the total’s last digit is 0 (or, put more formally, if the total modulo 10 is congruent to 0), the number is valid! So, wouldn't the number be valid?

What am I missing? Code below.

``````#include <stdio.h>
#include <cs50.h>

int countdigits(long num)
{
// count the number of digits
int count = 0;
// zero would return 0 digits - adjust
if (num == 0)
{
(count = 1);
}
else
{
while (num)
{
count++;
num = num / 10;
}
}
return (count);
} //end function

long digitnumber(long num, int pos)
{
// return the digit in the position pos
int digit = 0;
int tmp = 0;
if (pos > (countdigits(num)))
{
printf("Error, position %i is larger than number length (%i)\n", pos, countdigits(num));
exit(0);
}
else
{
for (int i = countdigits(num); i > 0; i--)
{
tmp = num % 10;
if (i == pos)
{
digit = tmp;
}
num = (num / 10);
//printf("Digit: %i i: %i num: %li\n", digit, i, num);
}
}
return digit;
}

//Begin Main Program
int main(void)
{

long s = -1;
while (s < 0)
{
s = get_long_long("Number: ");
}

// Step 1:
// Multiply every other digit by 2, starting with the
// number’s second-to-last digit, and then add those products' digits together.

int st = 0;
st = countdigits(s);
//printf("digits: %i\n", st);
int p = 0;

// Check Issuers depending on 2 first numbers

char *issuer = "INVALID";
if (digitnumber(s, 1) == 4)
{
issuer = "VISA";
}

p = (digitnumber(s, 1) * 10) + (digitnumber(s, 2));

if ((p == 34) || (p == 37))
{
issuer = "AMEX";
}

if ((p >= 51) && (p <= 55))
{
issuer = "MASTERCARD";
}

//printf("First 2 digits: %i\n", p);
//printf("Issuer: %s\n", issuer);

// Now let's determine if the number is valid

int j = countdigits(s) - 1;
float check = 0;
int l = 0;
while (j > 0)
{
l = digitnumber(s, j) * 2;
check = check + l;
if (l > 9)
{
check = check + digitnumber(l, 1) + digitnumber(l, 2) - l;
}
j = j - 2;
}

//printf("Check = %f\n", check);

j = countdigits(s);
while (j > 0)
{
check = check + (digitnumber(s, j));
j = j - 2;
}

//printf("Check = %f\n", check);

l = digitnumber(check, countdigits(check));
if (l != 0)
{
issuer = "INVALID";
}

//printf("last number = %i\n", l);
printf("%s\n", issuer);
}
``````