Problem description: The International Standard Book Number (ISBN) is a unique,

Question

Problem description:
    The International Standard Book Number (ISBN) is a unique, numerical commercial
    book identifier. The ISBN is 13 digits long if assigned after January 1, 2007.
    The last digit of the thireen-digit ISBN is called check digit, which is used to verify
    if an ISBN is a valid ISBN. The following describes the way to calculate the check digit.

    The calculation of an ISBN-13 check digit begins with the first 12 digits of the 13-digit ISBN
    (thus excluding the check digit itself).
    1) Each digit, from left to right, is alternately multiplied by 1 or 3,
    2) those products are summed and calculated modulo 10 to give a value ranging from 0 to 9.
    3) the value is subtracted from 10, that leaves a result from 1 to 10.
    4) A zero (0) replaces a ten (10), so, in all cases, a single check digit results.

    For example, the ISBN-13 check digit of 978-0-306-40615-? is calculated as follows:

    s = 9

Explanation / Answer

//First Blank will be filled by

int checkDigit(string);

bool isValidISBN(string);

//second blank will be filled by

isValid = isValidISBN(isbn);

//Third blan filled by

int checkDigit(string ISBN)

{

int res=0;

for(int i=0;i<12;i++)

{

if(i%2==0)

{

res+=(ISBN[i]-'0')*1;

}

else

{

res+=(ISBN[i]-'0')*3;

}

}

res = res%10;

res = 10 - res;

return res;

}

//forth spance will be filled by

bool isValidISBN(string ISBN)

{

if((ISBN[12]-'0')==checkDigit(ISBN))

return true;

else

return false;

}

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