credit card validator in c

credit.c

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


int main(void)
{
    // Declare and initialize cardname variable.
    // Change variable below to test credit card number
    long long cardnumber = 378282246310005;
 

    // Determine whether it has a valid number of digits
    int count = 0;
    long long digits = cardnumber;

    //checks for card length of 13, 15, or 16
    while (length > 0)
    {
        length = length/10;
        count++;
    }
    if ((count != 13) && (count != 15) && (count != 16))
    {
        printf("INVALID\n");
    }
    // create array size with the count of the number
    
    int number[count];
  
    // store each digit of card number in the array number
    for (int i = 0; i < count; i++)
    {
        number[i] = cardnumber % 10;
        cardnumber = cardnumber / 10;
    }

   // number = [1,8,8,1,8,8,8,8,8,8,8,8,2,1,0,4]


    int cardarray[count];
    for (int i = 1; i < count; i++)
    {
        cardarray[i] = number[i];
    }
    // multiply every other number in the array
    for (int i = 1; i < count; i+=2)
    {
        number[i] = number[i] * 2;
    }

    // cardarray = [8,8,1,8,8,8,8,8,8,8,8,2,1,0,4]
    // number = [1,16,8,2,8,16,8,16,8,16,8,16,2,2,0,4]


    int accumulator = 0;
    int sumdigit;
    
    if (count == 13)
    {
      for (int i = 0; i < count; i++)
      {
        sumdigit = (number[i] % 10) + (number[i]/10 % 10);
        accumulator = accumulator + sumdigit;
      }
      if (cardarray[12] == 4 && accumulator % 10 == 0)
      {
        printf("VISA\n");
      }
      else
      {
        printf("INVALID\n");
      }
    }
    if (count == 15)
    {
      for (int i = 0; i < count; i++)
      {
        sumdigit = (number[i] % 10) + (number[i]/10 % 10);
        accumulator = accumulator + sumdigit;
      }
      if (cardarray[14] == 3 && accumulator % 10 == 0 && (cardarray[13] == 4 || cardarray[13] == 7))
      {
        printf("AMEX\n");
      }
      else
      {
        printf("INVALID\n");
      }
    }
    if (count == 16)
    {
      for (int i = 0; i < count; i++)
      {
        sumdigit = (number[i] % 10) + (number[i]/10 % 10);
        accumulator = accumulator + sumdigit;
      }
      if (cardarray[15] == 4 && accumulator % 10 == 0)
      {
        printf("VISA\n");
      }
      else if (cardarray[15] == 5 && accumulator % 10 == 0 && (cardarray[14] == 1 || cardarray[14] == 2 || cardarray[14] == 3 || cardarray[14] == 4 || cardarray[14] == 5))
        {
            printf("MASTERCARD\n");
        }
      else
      {
        printf("INVALID\n");
      }
    }
  
    return 0;
} 

You did not initialize the variable "length"