Hacker pset1 credit.c always gives invalid?

Question

I tried to attempt the credit.c problem. My code is as follows:

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

int main(void)
{
    printf("Please enter your credit card number: ");
    long long credit = GetLongLong();
    char str[20];
    sprintf(str, "%lld", credit);
    int length = strlen(str);
    int sum1 = 0;
    int sum2 = 0;
    for (int i = length - 1; i >= 2; i = i - 2)
    {
        sum1 += (2 * (credit % 10^i));
    }
    for (int i = length; i >= 2; i = i - 2)
    {
        sum2 += (credit % 10^i);
    }
    int sum = sum1 + sum2;
    int fstwo = ((credit % (10^length)) + (credit % (10^(length - 1))));
    if (length == 15 && (fstwo == 34 || fstwo == 37) && sum % 10 == 0)
    {
        printf("AMEX\n");
    }
    else if (length == 16 && (fstwo >= 51 && fstwo <= 55) && sum % 10 == 0)
    {
        printf("MASTERCARD\n");
    }
    else if ((length == 13 || length == 16) && (credit % (10^length)) == 4 && sum % 10 == 0)
    {
        printf("VISA\n");
    }
    else
    {
        printf("INVALID\n");
    }
}

My logic is to convert a long long into a string via sprintf(), then use strlen() and for loops to detect every other digit in the long long. The first for loop starts from the second last digit and sums twice of each digit. The second one starts from the last digit and sums them as per normal. I used the modulo function to determine each digit in the long long. Finally, I implemented the various conditions in the if and else statements. However, all my tries so far have given me invalid results. Could someone please tell me what's wrong? I have absolutely no clue, thank you!

Answer 1

Firstly, you set the length of str to 20 and then determine the length of this wasting an instruction and memory.

Do your two for loops give the correct results? sum1 += (2 * (credit % 10^i)); will not give you the result you are looking for as the ^ operator is a bitwise exclusive or operator. if you want to the power of, you can use:

pow(10,i)

Also remember that the array is zero-indexed. So 0 is the first entry and 19 is the last entry.