implementing luhn algorithm using c#

Question

I am using following code to implement Luhn algorithm for credit card check in c# language but could not get the output to generate the check sum its showing validity: kindl

Answer 1

Here is a correct and fast implementation:

bool PassesLuhnCheck(string value)
{
    long sum = 0;

    for (int i = 0; i < value.Length; i++)
    {
        var digit = value[value.Length - 1 - i] - '0';
        sum += (i % 2 != 0) ? GetDouble(digit) : digit;
    }

    return sum % 10 == 0;

    int GetDouble(long i)
    {
        switch (i)
        {
            case 0: return 0;
            case 1: return 2;
            case 2: return 4;
            case 3: return 6;
            case 4: return 8;
            case 5: return 1;
            case 6: return 3;
            case 7: return 5;
            case 8: return 7;
            case 9: return 9;
            default: return 0;
        }
    }
}

Answer 2

I have tried this code which might help for other future folk:

    public string GenerateLuhnNumber(string baseNumber)
    {
        if (!double.TryParse(baseNumber, out double baseNumberInt))
            throw new InvalidWorkflowException($"Field contains non-numeric character(s) : {baseNumber}");

        var step2 = string.Empty;
        for (var index = baseNumber.Length - 1; index >= 0; index -= 2)
        {
            var doubleTheValue = (int.Parse(baseNumber[index].ToString())) * 2;

            if (doubleTheValue > 9)
                doubleTheValue = Math.Abs(doubleTheValue).ToString().Sum(c => Convert.ToInt32(c.ToString()));

            step2 = step2.Insert(0, (index != 0 ? baseNumber[index - 1].ToString() : "") + doubleTheValue);
        }
        var step3 = Math.Abs(Convert.ToDouble(step2)).ToString(CultureInfo.InvariantCulture).Sum(c => Convert.ToDouble(c.ToString())).ToString(CultureInfo.InvariantCulture);

        var lastDigitStep3 = Convert.ToInt32(step3[step3.Length - 1].ToString());
        string checkDigit = "0";

        if (lastDigitStep3 != 0)
            checkDigit = (10 - lastDigitStep3).ToString();

        return baseNumber + checkDigit;
    }

Answer 3

Here are some extension methods that compute a Luhn checkdigit, validate a number with a checkdigit, and add a checkdigit to a number. Tested in .NET 4.5.

There are extension methods for strings, ints, int64s and IList.

I got some ideas for this from rosettacode.org

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;

public static class CheckDigitExtension
{
    static readonly int[] Results = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

    #region extension methods for IList<int>

    /// <summary>
    /// For a list of digits, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The list of digits for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static int CheckDigit(this IList<int> digits)
    {
        var i = 0;
        var lengthMod = digits.Count%2;
        return (digits.Sum(d => i++ % 2 == lengthMod ? d : Results[d]) * 9) % 10;
    }

    /// <summary>
    /// Return a list of digits including the checkdigit
    /// </summary>
    /// <param name="digits">The original list of digits</param>
    /// <returns>the new list of digits including checkdigit</returns>
    public static IList<int> AppendCheckDigit(this IList<int> digits)
    {
        var result = digits;
        result.Add(digits.CheckDigit());
        return result;
    }

    /// <summary>
    /// Returns true when a list of digits has a valid checkdigit
    /// </summary>
    /// <param name="digits">The list of digits to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this IList<int> digits)
    {
        return digits.Last() == CheckDigit(digits.Take(digits.Count - 1).ToList());
    }

    #endregion extension methods for IList<int>

    #region extension methods for strings

    /// <summary>
    /// Internal conversion function to convert string into a list of ints
    /// </summary>
    /// <param name="digits">the original string</param>
    /// <returns>the list of ints</returns>
    private static IList<int> ToDigitList(this string digits)
    {
        return digits.Select(d => d - 48).ToList();
    }

    /// <summary>
    /// For a string of digits, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The string of digits for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static string CheckDigit(this string digits)
    {
        return digits.ToDigitList().CheckDigit().ToString(CultureInfo.InvariantCulture);
    }

    /// <summary>
    /// Return a string of digits including the checkdigit
    /// </summary>
    /// <param name="digits">The original string of digits</param>
    /// <returns>the new string of digits including checkdigit</returns>
    public static string AppendCheckDigit(this string digits)
    {
        return digits + digits.CheckDigit(); 
    }

    /// <summary>
    /// Returns true when a string of digits has a valid checkdigit
    /// </summary>
    /// <param name="digits">The string of digits to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this string digits)
    {
        return digits.ToDigitList().HasValidCheckDigit();
    }

    #endregion extension methods for strings

    #region extension methods for integers

    /// <summary>
    /// Internal conversion function to convert int into a list of ints, one for each digit
    /// </summary>
    /// <param name="digits">the original int</param>
    /// <returns>the list of ints</returns>
    private static IList<int> ToDigitList(this int digits)
    {
        return digits.ToString(CultureInfo.InvariantCulture).Select(d => d - 48).ToList();
    }

    /// <summary>
    /// For an integer, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The integer for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static int CheckDigit(this int digits)
    {
        return digits.ToDigitList().CheckDigit();
    }

    /// <summary>
    /// Return an integer including the checkdigit
    /// </summary>
    /// <param name="digits">The original integer</param>
    /// <returns>the new integer including checkdigit</returns>
    public static int AppendCheckDigit(this int digits)
    {
        return digits * 10 + digits.CheckDigit();
    }

    /// <summary>
    /// Returns true when an integer has a valid checkdigit
    /// </summary>
    /// <param name="digits">The integer to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this int digits)
    {
        return digits.ToDigitList().HasValidCheckDigit();
    }

    #endregion extension methods for integers

    #region extension methods for int64s

    /// <summary>
    /// Internal conversion function to convert int into a list of ints, one for each digit
    /// </summary>
    /// <param name="digits">the original int</param>
    /// <returns>the list of ints</returns>
    private static IList<int> ToDigitList(this Int64 digits)
    {
        return digits.ToString(CultureInfo.InvariantCulture).Select(d => d - 48).ToList();
    }

    /// <summary>
    /// For an integer, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The integer for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static int CheckDigit(this Int64 digits)
    {
        return digits.ToDigitList().CheckDigit();
    }

    /// <summary>
    /// Return an integer including the checkdigit
    /// </summary>
    /// <param name="digits">The original integer</param>
    /// <returns>the new integer including checkdigit</returns>
    public static Int64 AppendCheckDigit(this Int64 digits)
    {
        return digits * 10 + digits.CheckDigit();
    }

    /// <summary>
    /// Returns true when an integer has a valid checkdigit
    /// </summary>
    /// <param name="digits">The integer to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this Int64 digits)
    {
        return digits.ToDigitList().HasValidCheckDigit();
    }

    #endregion extension methods for int64s
}

Here are some XUnit test cases that show how the extension methods work.

public class CheckDigitExtensionShould
{
    [Fact]
    public void ComputeCheckDigits()
    {
        Assert.Equal(0, (new List<int> { 0 }).CheckDigit());
        Assert.Equal(8, (new List<int> { 1 }).CheckDigit());
        Assert.Equal(6, (new List<int> { 2 }).CheckDigit());

        Assert.Equal(0, (new List<int> { 3, 6, 1, 5, 5 }).CheckDigit());
        Assert.Equal(0, 36155.CheckDigit());
        Assert.Equal(8, (new List<int> { 3, 6, 1, 5, 6 }).CheckDigit());
        Assert.Equal(8, 36156.CheckDigit());
        Assert.Equal(6, 36157.CheckDigit());
        Assert.Equal("6", "36157".CheckDigit());
        Assert.Equal("3", "7992739871".CheckDigit());
    }

    [Fact]
    public void ValidateCheckDigits()
    {
        Assert.True((new List<int> { 3, 6, 1, 5, 6, 8 }).HasValidCheckDigit());
        Assert.True(361568.HasValidCheckDigit());
        Assert.True("361568".HasValidCheckDigit());
        Assert.True("79927398713".HasValidCheckDigit());
    }

    [Fact]
    public void AppendCheckDigits()
    {
        Console.WriteLine("36156".CheckDigit());
        Console.WriteLine("36156".AppendCheckDigit());
        Assert.Equal("361568", "36156".AppendCheckDigit());
        Assert.Equal("79927398713", "7992739871".AppendCheckDigit());
    }
}

Answer 4

Compact Luhn check:

public static bool Luhn(string digits)
{
    return digits.All(char.IsDigit) && digits.Reverse()
        .Select(c => c - 48)
        .Select((thisNum, i) => i % 2 == 0
            ? thisNum
            :((thisNum *= 2) > 9 ? thisNum - 9 : thisNum)
        ).Sum() % 10 == 0;
}

Fiddle: https://dotnetfiddle.net/CCwE48

Answer 5

This one will do it I believe:

static void Main(string[] args)
    {
        string number = "1762483";
        int digit = 0;
        int sum = 0;

        for (int i = 0; i <= number.Length - 1; i++)
        {

            if (i % 2 == 1)
            {
                digit = int.Parse(number.Substring(i, 1));
                sum += DoubleDigitValue(digit);

                Console.WriteLine(digit);
            }
            else
            {
                digit = int.Parse(number.Substring(i, 1));
                sum += digit;
            }

        }
        Console.WriteLine(sum);
        if (sum % 10 == 0)
        {
            Console.WriteLine("valid");
        }
        else
        {
            Console.WriteLine("Invalid");
        }
    }
    static int DoubleDigitValue(int digit)
    {
        int sum;
        int doubledDigit = digit * 2; 
        if (doubledDigit >= 10)
        {
            sum = 1 + doubledDigit % 10;
        } else
        {
            sum = doubledDigit; 
        }
        return sum; 
    }

Answer 6

These are my methods for validating and calculating the last digit. To validate a number simply check that the result of the first method is 0;

private int LuhnChecksum(string input)
    {
        var length = input.Length; 
        var even = length % 2; 
        var sum = 0;

        for (var i = length - 1; i >= 0; i--)  
        {
            var d = int.Parse(input[i].ToString());
            if (i % 2 == even)
                d *= 2;
            if (d > 9)
                d -= 9;
            sum += d;
        }
        return sum % 10;
    }

    private int LuhnCalculateLastDigit(string input) 
    {
        var checksum = LuhnChecksum(input + "0");
        return checksum == 0 ? 0 : 10 - checksum;
    }