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public class Main {

    public static void main(String[] args) {
        String str = "3-86680-193";
        String str2 = str + "-0";
        System.out.println( str );
        System.out.println( checksum10( str ) );
        System.out.println( str2 + " : " + checksum10valid( str2 ) );
    }

    public static char checksum10( String str ) {
        int i, sum = 0, count = 0;
        char c;

        for( i = 0; i < str.length(); i++ ) {
            c = str.charAt( i );
            if ( !Character.isDigit( c ) )
                continue;

            count++;
            if ( count > 9 )
                break;
            sum += count * Character.getNumericValue( c );
        }

        if ( ( i < str.length( )-1 ) || ( count != 9 ) )
            System.out.println( "Error" );

        sum %= 11;
        return( sum == 10 ? 'X' : Character.forDigit( sum, 10 ) );
    }

    public static boolean checksum10valid( String str ) {
        char c = str.charAt( str.length()-1 );

        return( c == checksum10( str.substring( 0, str.length()-1 ) ) );
    }

    public static char checksum13( String str ) {
        int i, sum = 0, count = 0;
        char c;

        for( i = 0; i < str.length(); i++ ) {
            c = str.charAt( i );
            if ( !Character.isDigit( c ) )
                continue;

            count++;
            if ( count > 12 )
                break;
            if ( count % 2 == 0 )
                sum += 3 * Character.getNumericValue( c );
            else
                sum += Character.getNumericValue( c );

        }

        if ( ( i < str.length( )-1 ) || ( count != 12 ) )
            System.out.println( "Error" );

        sum = ( 10 - sum%10 ) % 10;
        return( Character.forDigit( sum, 10 ) );
    }

    public static boolean checksum13valid( String str ) {
        char c = str.charAt( str.length()-1 );

        return( c == checksum13( str.substring( 0, str.length()-1 ) ) );
    }
}

                

def checkISBN10(s):
    l = s.split('-')
    ss = ''.join(l)
    print ss
    sum =0
    for i in range (len(ss)):
        sum+=(i+1)*int(ss[i])
    p = sum %11
    if p == 10: 
        return 'x'
    return str(p)

def checkISBN13(s):
    l = s.split('-')
    ss = ''.join(l)
    print ss
    sum =0
    for i in range (6):
        sum+=int(ss[i])
        sum+=3*int(ss[i])
    p = sum %10
    return str(p)

              

print checkISBN10('0-13-110362')
print checkISBN10('3-86680-193')
print checkISBN10('3-446-13878')
print checkISBN13('978-1-56619-909-4')
                

char berechnePruefziffer(String isbn10OhnePruefziffer) {
     int sum = 0;
     int factor = 1;
     for(char c : isbn10OhnePruefziffer.toCharArray()) {
        int  ziffernWert = c - '0';
        sum    = sum + factor * ziffernWert;
        factor = factor + 1;
     }
     int p = sum % 11;
     if(10 == p) {
         return 'X';
     } else {
         char ziffer = (char) (p + '0');
         return ziffer;
     }
  }
    
                

Lösung von: Philipp G. Freimann (BBW (Berufsbildungsschule Winterthur) https://www.bbw.ch)

function isbnChecksum(input) {
  input = input.replace(/[ -]/g, '').trim().split('');
  for (let i in input) input[i] = parseInt(input[i]);

  switch (input.length) {
    case 9 : return getCs10();
    case 10: return checkCs10();
    case 12: return getCs13();
    case 13: return checkCs13();
  }

  function getCs10() {
    let c = 0;
    for (i = 0; i <= 8; i++) c += (i+1) * input[i];
    c = c % 11;
    return (c == 10) ? 'X' : c;
  }

  function checkCs10() {
    return input[input.length-1] == getCs10();
  }

  function getCs13() {
    let c = 0;
    for (let i = 1; i <= 11; i = i+2) c += input[i];
    c *= 3;
    for (i = 0; i <= 10; i += 2) c += input[i];
    return (10-(c % 10)) % 10;
  }

  function checkCs13() {
    return input[input.length-1] == getCs13();
  }
}

// ausgabe
console.log( isbnChecksum("0-13-110362") );
console.log( isbnChecksum("3-89864-117-1") );
console.log( isbnChecksum("978-3-86680-192") );
console.log( isbnChecksum("978-3-86680-192-9") );
                

Lösung von: Lisa Salander (Heidi-Klum-Gymnasium Bottrop)

// NET 6.x | C# 10.x | VS-2022
using System.Text.RegularExpressions;
static bool IsIsbn(string s) {
    s = s.Replace("-", "").Replace(" ", "").ToUpper();
    if (new Regex(@"\b\d{9}[0-9X]\b").IsMatch(s)) {
        var i = 10;
        var r = s.Select(x => x is 'X' ? 10 : x - '0');
        return (r.Take(9).Select(x => x * i--).Sum() + r.TakeLast(1).FirstOrDefault()) % 11 == 0;
    }
    if (new Regex(@"\b\d{13}\b").IsMatch(s)) {
        var sum = Enumerable.Range(0, s.Length - 1).Select(x => (s[x] - '0') * Math.Pow(3, x % 2)).Sum();
        return (int)Math.Ceiling(sum / 10) * 10 - sum == s[^1] - '0';
    }
    return false;
}
Console.WriteLine(IsIsbn("3-8931-9301-4"));
Console.WriteLine(IsIsbn("40 03 30 10 18 39 8"));
                

Lösung von: Jens Kelm (@JKooP)

// C++ 14 | VS-2022
#include <iostream>
#include <vector>
#include <algorithm>

enum class ISBN_Type { isbn_10 = 8, isbn_13 = 11 };

struct ISBN { std::string num; ISBN_Type typ; };

const auto get_isbn_check_num(const ISBN& isbn) {
    auto sum{ 0 }, counter{ 0 };
    for (size_t i{ 0 }; i < isbn.num.length(); ++i) {
        if (counter > int(isbn.typ)) return -1;
        if (std::isdigit(isbn.num[i]))
            sum += (isbn.num[i] - '0') * (isbn.typ == ISBN_Type::isbn_10 ? ++counter : pow(3, counter++ % 2));
    }
    return (isbn.typ == ISBN_Type::isbn_10 ? (sum % 11 == 10 ? 'X' : sum % 11) : ((sum + 9) / 10) * 10 - sum);
}

const auto is_isbn_num(const ISBN& isbn) {
    const auto cn{ std::find_if(isbn.num.rbegin(), isbn.num.rend(), [](auto x) {return std::isdigit(x); }) };
    if (cn != isbn.num.rend())
        return (*cn - '0') == get_isbn_check_num({isbn.num.substr(0, isbn.num.length()-2), isbn.typ});
    return false;
}

int main() {
    // Prüfziffern ermitteln
    const std::vector<ISBN>isbn_1 {
        {"0-13-110362", ISBN_Type::isbn_10},        // 8
        {"3-89864-117", ISBN_Type::isbn_10},        // 1
        {"978381582086", ISBN_Type::isbn_13},       // 5
        {"97 83 82 73 17 10", ISBN_Type::isbn_13},  // 0
        {"4003301018392", ISBN_Type::isbn_13}       // -1 (keine ISBN!)
    };

    for (const auto& i : isbn_1)
        std::cout << i.num << " - > " << get_isbn_check_num(i) << "\n";

    // Prüfen, ob es sich um eine ISBN-Nummer handelt (inkl. Prüfziffer)
    ISBN isbn_2{ "0-13-110362-8", ISBN_Type::isbn_10 };
    std::cout << isbn_2.num << " -> " << (is_isbn_num(isbn_2) ? "Wahr" : "Falsch\n");
}
                

Lösung von: Jens Kelm (@JKooP)