error at while loop cs50 Pset1 credit

Question

I am getting an error at while loop it says

error: '&&' within '||' [-Werror,-Wlogical-op-parentheses]
       while((credit % 10 >= 0) && (i < 15) || (i < 16))

here is my code for the credit also please check if my logic is correct.

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

int main(void)
{
    int count=0;
    long int credit;
    int sum = 0;
    int sum1 = 0;
    int i=0;
    int t,s;
    do
    {
        credit = get_long("Number: ");
    }
    while(credit < 1);

    while((credit%10) > 0)
    {
        count++;
    }

    if((count != 15) || (count != 16))
    {
        printf("INVALID\n");
        return 0;
    }
    else
    {
       while((credit % 10 >= 0) && (i < 15) || (i < 16))
       {
           i++;
           if(i%2 != 0)
           {
               sum = sum + (credit % 10);
           }
           else
           {
               t = 2 * (credit % 10);
               while(t%10 >= 0){
                   sum1 = sum1 + (t%10);
               }
           }
       }
        s = sum + sum1;
        if (s%10 == 0)
        {
            printf("valid credit\n");
        }
        else
        {
            printf("invalid credit\n");
        }
    }
}

Answer 1

The compiler is actually giving a warning, but the compiler flags are set up to treat all warnings as errors. It's still best to eliminate all warnings.

Here's the reason. For clarity, let's simplify the expression to the essential parts.

   while((a) && (b) || (c))

The compiler isn't sure about how these logical operators should be combined. This expression could mean (a && b) || c or it could mean a && (b || c) It's saying that the expression is ambiguous. You could apply parentheses, as in one of the two examples, to resolve this.

However, let's look at the actual statement,

   while((credit % 10 >= 0) && (i < 15) || (i < 16))

If the form is a && (b || c), then the tests can be simplified. If i < 15 is true, then i < 16 must be true, so i < 15 is redundant. If the form is (a && b) || c, then its absolutely critical that parentheses be added to make it unambiguous. These reasons are probably why the compiler gave a warning.

If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)

Answer 2

You should add brackets, so it'll look like this:

while ((credit % 10 >= 0) && ((i < 15) || (i < 16)))

or this:

while (((credit % 10 >= 0) && (i < 15)) || (i < 16))

(It should actually look like the first one, because 1) I did this pset before, and 2) I read your code

You should also use style50. (No offence) The code is rather messy.

With style50, your code will like what it is here