Week-1 problem set credit

Question

I have started taking the CS5 lessons and while I run my code for the week-1 credit problem, the terminal always shows the output as "INVALID" or anything that I insert in the second last printf command. Can someone help me find the error?

Thanks

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

int main(void)
{
    //getting the credit card number
    long long cn=0;
    do
    {
        cn=get_long_long("Enter your credit card number for validity check: ");
    }
    while(cn<1);

    //determining the length of the credit card number
    int length=0;
    long long cn_temp=cn;
    while(cn_temp>0)
    {
        cn_temp=cn_temp/10;
        length++;
    }

    printf("The number of digits entered is: %i\n" , length);

    //Storing the digits of the number in an array
    int digits[length];
    for(int i=0; i<length; i++)
    {
        digits[i]=cn%10;
        cn/=10;
    }

    //Sum of odd and even places from the right
    int sum_even=0;
    for(int i=length-2; i>=0; i=i-2)
    {
        int even_places=digits[i]*2;
        sum_even+=(even_places%10)+(even_places/10);
    }
    printf("Sum of even places from the right are: %i\n", sum_even);

    //adding the alernate sum to sum of odd places integers
    int sum_odd=0;
    for(int i=length-1; i>=0; i=i-2)
    {
        int odd_places=digits[i];
        sum_odd+=odd_places;

    }
    printf("Sum of odd places from the right are: %i\n", sum_odd);

    // Sum of even and odd places digits
    int sum_total=sum_even+sum_odd;
    int remainder=sum_total%10;

    if(remainder==0)
    {
        if(length==15 && digits[0]==3 && (digits[1]==4 ||digits[1]==7))
        {
            printf("AMEX\n");
            return 0;
        }
        else if(length==16 && digits[0]==5 && (digits[1]==1 ||digits[1]==2||digits[1]==3||digits[1]==4||digits[1]==5))
        {
            printf("MASTERCARD\n");
            return 0;
        }
        else if((length==13 || length==16) && digits[0]==4)
        {
            printf("VISA\n");
            return 0;
        }
        else
        {
            printf("INVALID\n");
            return 0;
        }

    }
    else
    {
        printf("INVALID\n");
        return 0;
    }

}

Answer 1

The problem lies in the data stored in the digits[] array. Try printing out the contents of each element in digits, ie, digits[0], digits[1] etc. Also, make sure that you print out ALL of the elements. Don't assume that you know how big the array is! ;-)

Insert this as line 31:

        printf("digits[%i]=%i  cn = %lld \n",i, digits[i], cn);

I'm sure that once you do this, you'll see the problem immediately.

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