CS50 Luhn's Algorithm in C

I'm new to C and I'm doing CS50. I can't get my code to work. I'm almost positive the for statement is correct. The sum given by the for statement is correct.

I think it's something to do with the (floor(ccNumber / pow(10,13) == 34) in the if statements.

Explanation of Luhn's Algorithm: https://cs50.harvard.edu/x/2020/psets/1/credit/

Sample credit card numbers for testing: https://www.freeformatter.com/credit-card-number-generator-validator.html

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

void credit(long ccNumber);

int main(void)
{
    long ccNumber = 0;
    credit(ccNumber);
}

void credit(long ccNumber)
{

    do

    {
        ccNumber = get_long("Enter a credit card number: ");
    }

    while (ccNumber < 0);

    {
        int sum = 0;
        long ccNumberFormat = ccNumber;
        int nDigits = floor(log10(ccNumberFormat)) + 1;
        int parity = nDigits % 2; // if parity % 2 == 0 then even number of digits, otherwise odd number of digits


        for (int i = nDigits; i >= 0; i--) {
            int digit = ccNumberFormat % 10;

            if (i % 2 != parity) { // even number credit card digits will be multiplied by 2 every even number (starting from 0)
                digit = digit * 2;
                // printf("%d\n", sum);
            }
            if (digit > 9) {
                digit = digit - 9;
            }

            sum = sum + digit;
            ccNumberFormat /= 10;
            printf("%d\n", sum);
        }

        if (sum % 10 == 0 && nDigits == 15 && (floor(ccNumber / pow(10,13) == 34) || (floor(ccNumber / pow(10,13) == 37))))
        {
            printf("%s\n", "AMEX");
        }
        else if (sum % 10 == 0 && (nDigits == 13 || nDigits == 16) && (floor(ccNumber / pow(10,12) == 4) || floor(ccNumber / pow(10,15) == 4)))
        {
            printf("%s\n", "VISA");
        }
        else if (sum % 10 == 0 && nDigits == 16 && ((floor(ccNumber / pow(10,14) >= 51) || (floor(ccNumber / pow(10,14) <= 55)))))
        {
            printf("%s\n", "MASTERCARD");
        }
    }
}

Pseudocode I more or less followed on Wikipedia::

function checkLuhn(string purportedCC) {
    int sum := integer(purportedCC[length(purportedCC)-1])
    int nDigits := length(purportedCC)
    int parity := nDigits modulus 2
    for i from 0 to nDigits - 2 {
        int digit := integer(purportedCC[i])
        if i modulus 2 = parity
            digit := digit × 2
        if digit > 9
            digit := digit - 9
        sum := sum + digit
    }
    return (sum modulus 10) = 0
}

A credit card number is a big number, are you sure long is enough and you do not need long long

Out of that in your expressions like floor(ccNumber / pow(10,13) == 34) the ')' is wrongly placed and you wanted floor(ccNumber / pow(10,13)) == 34 and of course the same for others :

    if (sum % 10 == 0 && nDigits == 15 && ((floor(ccNumber / pow(10,13)) == 34) || (floor(ccNumber / pow(10,13)) == 37)))
    {
        printf("%s\n", "AMEX");
    }
    else if (sum % 10 == 0 && (nDigits == 13 || nDigits == 16) &&((floor(ccNumber / pow(10,12)) == 4) || (floor(ccNumber / pow(10,15)) == 4)))
    {
        printf("%s\n", "VISA");
    }
    else if (sum % 10 == 0 && nDigits == 16 && ((floor(ccNumber / pow(10,14)) >= 51) || (floor(ccNumber / pow(10,14)) <= 55)))
    {
        printf("%s\n", "MASTERCARD");
    }

but in

    else if (sum % 10 == 0 && (nDigits == 13 || nDigits == 16) &&((floor(ccNumber / pow(10,12)) == 4) || (floor(ccNumber / pow(10,15)) == 4)))

you take the risk to have a bad result, you need to check (floor(ccNumber / pow(10,12)) == 4) only if nDigits == 13 and (floor(ccNumber / pow(10,15)) == 4) only if nDigits == 16 and you can simplify to have :

    else if (sum % 10 == 0 && (nDigits == 13 || nDigits == 16) &&(floor(ccNumber / pow(10,nDigits-1)) == 4))

So finally :

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

void credit(long long ccNumber);

int main(void)
{
  long long cc[] = { 4532057997187363ll, // visa
                     4485661945778178ll, // visa
                     2720995573736457ll, // MasterCard
                     2720998284576493ll, // MasterCard
                     375137447049450ll, // amex
                     378572901284556ll, // amex
  };

 for (int i = 0; i != sizeof(cc)/sizeof(*cc); ++i)
    credit(cc[i]);
  
  return 0;
}

void credit(long long ccNumber)
{
  int sum = 0;
  long long ccNumberFormat = ccNumber;
  int nDigits = floor(log10(ccNumberFormat)) + 1;
  int parity = nDigits % 2; // if parity % 2 == 0 then even number of digits, otherwise odd number of digits
  
  
  for (int i = nDigits; i >= 0; i--) {
    int digit = ccNumberFormat % 10;
    
    if (i % 2 != parity) { // even number credit card digits will be multiplied by 2 every even number (starting from 0)
      digit = digit * 2;
      // printf("%d\n", sum);
    }
    if (digit > 9) {
      digit = digit - 9;
    }
    
    sum = sum + digit;
    ccNumberFormat /= 10;
    //printf("%d\n", sum);
  }
  
  if (sum % 10 == 0 && nDigits == 15 && ((floor(ccNumber / pow(10,13)) == 34) || (floor(ccNumber / pow(10,13)) == 37))) {
      printf("%lld %s\n", ccNumber, "AMEX");
    }
  else if (sum % 10 == 0 && (nDigits == 13 || nDigits == 16) &&(floor(ccNumber / pow(10,nDigits-1)) == 4)) {
    printf("%lld %s\n", ccNumber, "VISA");
  }
  else if (sum % 10 == 0 && nDigits == 16 && ((floor(ccNumber / pow(10,14)) >= 51) || (floor(ccNumber / pow(10,14)) <= 55))) {
    printf("%lld %s\n", ccNumber, "MASTERCARD");
  }

}

Compilation and execution:

pi@raspberrypi:/tmp $ gcc -Wall f.c -lm
pi@raspberrypi:/tmp $ ./a.out
4532057997187363 VISA
4485661945778178 VISA
2720995573736457 MASTERCARD
2720998284576493 MASTERCARD
375137447049450 AMEX
378572901284556 AMEX
pi@raspberrypi:/tmp $ 

note also to do floating point computation is a risk, you can do all only using long long