Credit from CS50 2018 week 1 walkthrough

Question

Hi i am a beginner and i have been trying to solve this set but with no avail.

I tried to use what was taught in the first lesson and did search for some help online when i encounter errors. However, i am still unable to execute the code properly. What may have gone wrong?

The question apparently require me to check if the credit card number is valid. First I want to make sure the credit card number consist of 16digits, then I tried to identify the respective digits. I then multiply the alternate numbers starting from second last digit by 2. After which I tried to identify any 2digits value (e.g 18) from the multiplied result and sum it (eg 1+8). Then add the sum of the another set of alternate digits with the sum of the multiplied amount. And if the last digit of the sum=0, then it should return approve.

I managed to get no error for my codes but it keeps giving me “disapprove” even if the credit card number should return “approve” please help thank you.

this is my code:

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


            int count=0;//no. of digits


            int sum1=0;
            int sum2;
            int sum3;
            int main(void)
            {
            long long cc = 0;
            do
            {
            printf ("please input your credit card no. \n cc.:"); // prompt
            cc=get_long_long();}
            while (cc< 0);

            //determine if it has valid number of digits
             long long digits = cc;
              while (digits>0)
             {
             digits/=10;
             count++;
        }

            if (count!=16)
            {printf("invalid\n");

          }
        else{


    for(int i=1;i<=count; i++)
    {       double a=0;
            double b=0;
            a+= pow(10,i);
            b+= pow(10,(i-1));
            long long c;
            long long d;
            c=a;//convert double to int so % can be used
            d=b;

        if(i%2==0)//identify even position digits
         {

           int even_digits;
            int xsum=0;
            even_digits=2*(((cc%c)-(cc%d))/d); //2* even position digits

            if(even_digits>9)//identify 2 digits values after 2* e.g. 18
            {
                xsum+=(even_digits/10)+(even_digits%10);//sum the 2 value digits e.g. 1+8=9
            }
            else
            { sum1+=even_digits; } //sum of all the even position digits that has only 1 value
            sum2=sum1+ xsum;//sum of all the even position digits

        }
        else
        {

            int odd_digits=0;
            odd_digits=((cc%c)-(cc%d))/d;//identify all value of odd position digits
            sum3= sum3+odd_digits;//sum all values on odd position
        }}

            int sum4=0;
             sum4 = sum3+sum2; //sum of even and odd position digits

    if(sum4%10==0)
    {
        printf("approve\n");
    }
    else
    {
        printf("disapprove\n");
    }

    }

}

Answer 1

There are a number of issues.

First, the requirement says ... return INVALID, not invalid or disapprove. "approve" is also not going to work. If a card is good, it should return the name of the card - VISA, AMEX or MASTERCARD.

Card number lengths are 13, 15 and 16, not just 16. And, you need to know which type of card goes with which length.

The instructions are very specific. This is one of the hidden lessons of the early psets. If they say something specific, do exactly what it says, not something close. This is particularly important in a team programming environment where many people are working on parts of a program that have to fit together correctly later. Close doesn't cut it.

Most importantly, the accumulator math for the even digits in the code contains two problems. sum1 doesn't reset between passes, so its value from the last time even_digits was <= 9, it retains its value and is added to the calculation when even_digits > 9. Next, sum2 is not adding back its own current value to accumulate the correct total for even digits.

Also, there are 3 vars, sum1, sum2, and sum3, that are declared outside of main. This is called declaring a global variable. Vars should never be declared as globals unless there's a really, really good reason. That's in a later lesson.

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