What's wrong with my Credit code? works for some and doesn't for the others

Question

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

int main(void)
{
long long cardNumber;
char cardNumberStr[16]; 
char cardNumberArray[16];
int i, numberOfDigits;
int sum = 0;
do
{
    printf("Enter your card number.\n");
    cardNumber = get_long_long();
}while(cardNumber < 0); //Check if card number is out of range, less than 0


//Calculates the number of digits in the cardNumber
numberOfDigits = log10(cardNumber) + 1;

//If the number of digits is not equal to 13 or 15 or 16 (VISA, MASTERCARD, AMEX) then it's an invalid card number
if(numberOfDigits != 13 && numberOfDigits != 15 && numberOfDigits != 16)
{
    printf("INVALID\n");
    return 0;
}

//Convert the number to a string to be able to deal with each digit easily
sprintf(cardNumberStr, "%lld", cardNumber);
//Convert each char in the string to an integer number and store it in an array for easier calculations
for(i = 0; i < numberOfDigits; i++)
{
    cardNumberArray[i] = cardNumberStr[i] - '0';
}

for(i = 0; i < numberOfDigits; i++)
{
    if(i % 2 == 0) //If the order of the number is even AKA not supposed to be multiplied by 2 then we should just add it to the sum
    {
        sum += cardNumberArray[i];
    }
    else //If the order of the number is odd then this number is supposed to be multiplied by 2
    {
        /*If the number times 2 is greater than or equal 10 then it consists of 2 parts which should be divided then summed*/
        if((cardNumberArray[i] * 2) >= 10) 
            /*The 2nd digit should be summed hence the moduluo and since the number would never exceed (2 * 9 = 18) then a 1 is added*/
            sum += ((cardNumberArray[i] * 2) % 10) + 1;
        }
        else
        {
            /*If the number is less than 10 then it should be multiplied by 2 and added to the sum immediatly*/
            sum += (cardNumberArray[i] * 2);
        }
    }
}

//If the last digit of the sum is 0 then the credit card number is valid
if(sum % 10 == 0)
{
    if(cardNumberArray[0] == 3) // Since all AMEX credit cards start with 3
    {
        printf("AMEX\n");
    }
    else if(cardNumberArray[0] == 5) // Since all MASTERCARD credit cards start with 5
    {
        printf("MASTERCARD\n");
    }
    else if(cardNumberArray[0] == 4) // Since all VISA credit cards start with 4
    {
        printf("VISA\n");
    }
}
else
{
    printf("INVALID\n");
}
 }

Here is the checks50 results https://cs50.me/checks/713f559f14249bcf264dda1df6e9c24ea6c7b13e

Answer 1

I think you should start by being much more specific in your checks.

For instance: In your code, I see:

if(cardNumberArray[0] == 3) // Since all AMEX credit cards start with 3

while the specifications say:

American Express numbers all start with 34 or 37

Also, your code says:

else if(cardNumberArray[0] == 5) // Since all MASTERCARD credit cards start with 5

while the specifications say:

MasterCard numbers all start with 51, 52, 53, 54, or 55

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By the way: I see inline comments. Did style50 credit.c fail to give you warnings for those inline comments?

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Finally, to quote Cliff B:

If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)