CODEWITHSUNDEEP
c++
Alpha
Alpha

Reputation: 77

How do I use the char value for string?

Sorry if this is a dumb question, this is my first coding class.

If the checksum is 10, the last digit is denoted as X according to the ISBN-10 convention. Write a program that prompts the user to enter the first 9 digits and displays the 10-digit ISBN (including leading zeros). Your program should read the input as an integer.

A sample run should look something like this:

Enter the first nine digits of the ISBN: 013601267
The ISBN-10 number is: 0136012671

I have successfully made a program that can do this but using int value for all nine numbers. Unfortunately, this required the user to input every number separately.

So what I am trying to do now is use use a string ISBN so that I can target individual sections ie. isbn[0] * 1 + isbn[1] * 2...

I have also tried static_cast<char>(ISBN[0]) * 1 + static_cast<char>.... thinking it would do something and I get the same results.

string ISBN;
cout << "Enter the first nine digits of the ISBN as integer: ";
cin>>ISBN;

int n10 = (ISBN[0] * 1 + ISBN[1] * 2 + ISBN[2] * 3 + ISBN[3] * 4 + ISBN[4] * 5 + ISBN[5] * 6 + ISBN[6] * 7 + ISBN[7] * 8 + ISBN[8] * 9) % 11;
if (n10 == 10)
{
    cout << ISBN << "X" << endl;
}
else
{
    cout << ISBN << n10 << endl;
}

So when I input this number 013601267 I should get a 1 (0136012671) at the end instead I am getting a 5 (0136012675).

I think this is happening because it is giving me ASCII dec value instead of the char value.

Upvotes: 2

Views: 118

Answers (2)

Loki Astari
Loki Astari

Reputation: 264331

Four things you should check:

1: The size of the string is actually 9 characters.

if (ISBN.size() != 9) {
    // Error
}

Otherwise accessing elements that do not exist will cause an error in your program.

2: The digits do not start at value 0. In ASCII (or UTF-8) the digits start at 48. Therefore 48 => '0' 49 => '1' etc. But C++ guarantees all the digits are contiguous so as long as you know the first one you can subtract that and get a correct value. If you use '0' in an integer expression it will convert to the correct value. Thus to generate a number value from a char you should subtract this value from each digit before multiplying.

n10 = ((ISBN[0] - '0') * 1) + ((ISBN[1] - '0') * 2) + ...

3: But you should check the string is all digits.

for(auto x: ISBN) {
    if (!std::is_digit(x)) {
        // ERROR
    }
}

4: To print a string of 9 characters with leading zero you need to make sure you prep the stream coreectly:

std::cout << std::setw(9) << std::setfill('0') << number;

Or if the number is already in a string form that you know is 9 characters long you can simply use:

std::cout << ISBN;

So To output the correct 10 character number in your case:

std::cout << ISBN << ((n10 == 10) ? 'X' : ('0' + n10)) << "\n";

Upvotes: 1

NeilXu
NeilXu

Reputation: 31

  1. First you should check the size of string
if(ISBN.size() != 9){
    // ERROR
}
  1. You can use for to calculate 'n10'
int n10 = 0;
for(int i = 0; i < ISBN.size(); ++i){
    if(std::is_digit(x)){    // check digit
        // characters '0','1','2'... are not same as digits 0, 1, 2...
        // the value of '0' is 48, as shown here [ascii table][1] 
        n10 = (ISBN[i] - '0') * (i+1) + n10;
    } else {
        // ERROR
    }
}

Upvotes: 0

Related Questions