isbn, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            //"978-0-306-40615-?";
            var code = "97807 35623 87?";
            var isbn = new ISBN(code);
        }
    }
    
    public class ISBN
    {
        public ISBN(string code)
        {
            //prefix element, registration group, registrant, publication and check digit
            string str;// string holder
            int i = 0; // iterator for the looop
            int s = 0; // s is the checksum digit
            
            List<int> j = new List<int>();
            
            str = Regex.Replace(code, @"[?]","0"); // Replace '?' with '0'
            str = Regex.Replace(str, @"[^0123456789?]", string.Empty); // Remove non digits
            
            // Error check length
            if(str.Length != 10 && str.Length != 13)
                Console.Write("Error {0} of digits is invalid isbn\n", str.Length);

foreach(char c in str)
            {
                int n = Int32.Parse(c.ToString());
                //s = 9×1 + 7×3 + 8×1 + 0×3 + 3×1 + 0×3 + 6×1 + 4×3 + 0×1 + 6×3 + 1×1 + 5×3
                // =   9 +  21 +   8 +   0 +   3 +   0 +   6 +  12 +   0 +  18 +   1 +  15
                // = 93
                //   93 / 10 = 9 remainder 3
                //10 –  3 = 7
                if(IsEven(i))
                    s += n * 1;
                else
                    s += n * 3;
                
                
                
                //Console.WriteLine("Iseven -> " + i.ToString());
                //Console.Write("{0}\n", n.ToString());
                i++;
            }
            
            s = (10 - (s % 10)) % 10;
            
            Console.WriteLine("result $s == " + s.ToString());
            Console.Write("$str == {0}\n", str);
            
        }
        
        private bool IsOdd(int val) => val % 2 != 0;
        private bool IsEven(int val) => val % 2 == 0;
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,26 sec, absolute running time: 0,17 sec, cpu time: 0,16 sec, average memory usage: 18 Mb, average nr of threads: 4, absolute service time: 0,48 sec