问题
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: kindly help me.Thank you in advance
public class Program
{
private static void Main(string[]creditcard)
{
int sum = 0, d;
string num ="7992739871";
int a = 0;
for (int i = num.Length - 2; i >= 0; i--)
{
d = Convert.ToInt32(num.Substring(i, 1));
if (a % 2 == 0)
d = d * 2;
if (d > 9)
d -= 9;
sum += d;
a++;
}
if ((10 - (sum % 10) == Convert.ToInt32(num.Substring(num.Length - 1))))
Console.WriteLine("valid");
Console.WriteLine("sum of digits of the number" + sum);
}
}
回答1:
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());
}
}
回答2:
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
回答3:
Your algorithm is correct, but you're testing it wrong way.
I can see your sample input is from wiki page: Luhn algorithm. Difference is, they are calculating check digit for "7992739871X", where X is the check digit they're looking for. Your code validates number your already have!
Change your input to "79927398713" and it will mark it as correct number.
Update
OK, I see where is the problem. You're not taking this part of algorithm right:
From the rightmost digit, which is the check digit, moving left, double the value of every second digit;
Your code takes every other digit, but not necessary starting from most left digit. Try this code:
for (int i = 0; i < num.Length; i++)
{
d = Convert.ToInt32(num.Substring(num.Length - 1 - i, 1));
if (a % 2 == 0)
d = d * 2;
if (d > 9)
d -= 9;
sum += d;
a++;
}
var checkDigit = 10 - (sum % 10);
回答4:
You can do it very simply (reference),
public static bool Mod10Check(string creditCardNumber)
{
// check whether input string is null or empty
if (string.IsNullOrEmpty(creditCardNumber))
{
return false;
}
int sumOfDigits = creditCardNumber.Where((e) => e >= '0' && e <= '9')
.Reverse()
.Select((e, i) => ((int)e - 48) * (i % 2 == 0 ? 1 : 2))
.Sum((e) => e / 10 + e % 10);
return sumOfDigits % 10 == 0;
}
回答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;
}
回答6:
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;
}
回答7:
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;
}
}
}
回答8:
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;
}
来源：https://stackoverflow.com/questions/21249670/implementing-luhn-algorithm-using-c-sharp