Benzel - 6 months ago 42

C++ Question

I'm currently writing a program to give the user a checksum based off numbers from a part of an ISBN that they input. My calculation for the 13-digit ISBN is giving me the right answers, but for some reason, the calculation for the 10-digit is always giving me 11, no matter what I put in.

`cout << "Enter the digits one at a time, as if they were to be read from right to left." << endl;`

cin >> digit_one;

cin >> digit_two;

cin >> digit_three;

cin >> digit_four;

cin >> digit_five;

cin >> digit_six;

cin >> digit_seven;

cin >> digit_eight;

cin >> digit_nine;

checksum = (11 - (((10, 9, 8, 7, 6, 5, 4, 3, 2) * (digit_one, digit_two, digit_three, digit_four, digit_five, digit_six, digit_seven, digit_eight, digit_nine)) % 11));

cout << "The checksum for this ISBN is " << checksum << endl;

Is there something simple that I'm missing? Thanks for the help in advance.

Answer

You seem to misunderstand how operator `,`

works. `(2, 3)`

yields `3`

, not a tuple like it would in Python.

In the same vein `(2,3,4) * (1,2,3)`

is pretty much equivalent to `4 * 3`

.

So, in your code

```
checksum = (11 - (((10, 9, 8, 7, 6, 5, 4, 3, 2) * (digit_one, digit_two, digit_three, digit_four, digit_five, digit_six, digit_seven, digit_eight, digit_nine)) % 11))
```

is no different than

```
checksum = (11 - ((2 * digit_nine) % 11))
```

It's then safe to assume that your `digit_nine`

is `0`

¹ and that's why finally `checksum`

has the value of `11 - 0`

¹ or a multiplication of `11`

and not a digit at all