## How can I calculate a factorial in C# using a library call?

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I need to calculate the factorial of numbers up to around 100! in order to determine if a series of coin flip-style data is random, as per this Wikipedia entry on Bayesian probability. As you can see there, the necessary formula involves 3 factorial calculations (but, interestingly, two of those factorial calculations are calculated along the way to the third).

I saw this question here, but I'd think that integer is going to get blown out pretty quickly. I could also make a function that is more intelligent about the factorial calculation (ie, if I have 11!/(7!3!), as per the wiki example, I could go to (11*10*9*8)/3!), but that smacks of premature optimization to me, in the sense that I want it to work, but I don't care about speed (yet).

So what's a good C# library I can call to calculate the factorial in order to get that probability? I'm not interested in all the awesomeness that can go into factorial calculation, I just want the result in a way that I can manipulate it. There does not appear to be a factorial function in the Math namespace, hence the question.

You could try Math.NET - I haven't used that library, but they do list Factorial and Logarithmic Factorial.

**3 Different ways to calculate factorial in C# – Csharp Star,** This C# Program generates Factorial of the Number obtained from the user. 1. Using For Loop:. In this article, we will discuss different ways for calculating factorial in C#. Factorial of a number is obtained from the result of multiplying a series of descending natural numbers. This C# Program generates Factorial of the Number obtained from the user. 1. Using For Loop:

There has been a previous question on a similar topic. Someone there linked the Fast Factorial Functions web site, which includes some explanations of efficient algorithms and even C# source code.

**C# Sharp Exercises: Calculate the factorial of a given number ,** C# Sharp For Loop: Exercise-15 with Solution. Write a C# Sharp program to calculate the factorial of a given number. Pictorial Presentation:. To calculate a factorial in C#, you can use any of the following three ways − Calculate factorial with for loop Example. Live Demo. using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace factorial { class Test { static void Main(string[] args) { int i, res; int value = 5; res = value; for (i = value - 1; i >= 1; i--) { res = res * i; } Console

Do you want to calculate factorials, or binomial coefficients?

It sounds like you want to calculate binomial coefficients - especially as you mention 11!/(7!3!).

There may be a library that can do this for you, but as a (presumably) programmer visiting stack overflow there's no reason not to write one yourself. It's not too complicated.

To avoid memory overflow, don't evaluate the result until all common factors are removed.

**This algorithm still needs to be improved**, but you have the basis for a good algorithm here. The denominator values need to be split into their prime factors for the best result. As it stands, this will run for n = 50 quite quickly.

float CalculateBinomial(int n, int k) { var numerator = new List<int>(); var denominator = new List<int>(); var denominatorOld = new List<int>(); // again ignore the k! common terms for (int i = k + 1; i <= n; i++) numerator.Add(i); for (int i = 1; i <= (n - k); i++) { denominator.AddRange(SplitIntoPrimeFactors(i)); } // remove all common factors int remainder; for (int i = 0; i < numerator.Count(); i++) { for (int j = 0; j < denominator.Count() && numerator[i] >= denominator[j]; j++) { if (denominator[j] > 1) { int result = Math.DivRem(numerator[i], denominator[j], out remainder); if (remainder == 0) { numerator[i] = result; denominator[j] = 1; } } } } float denominatorResult = 1; float numeratorResult = 1; denominator.RemoveAll(x => x == 1); numerator.RemoveAll(x => x == 1); denominator.ForEach(d => denominatorResult = denominatorResult * d); numerator.ForEach(num => numeratorResult = numeratorResult * num); return numeratorResult / denominatorResult; } static List<int> Primes = new List<int>() { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 }; List<int> SplitIntoPrimeFactors(int x) { var results = new List<int>(); int remainder = 0; int i = 0; while (!Primes.Contains(x) && x != 1) { int result = Math.DivRem(x, Primes[i], out remainder); if (remainder == 0) { results.Add(Primes[i]); x = result; i = 0; } else { i++; } } results.Add(x); return results; }

I can estimate n = 110, k = 50 (returns 6x10^31) but cannot run n = 120, k = 50.

**C# factorial,** To calculate factorial in C#, you can use while loop and loop through until the number is not equal to 1.Here n is the value for which you want� Factorial of a number using the do-while loop in C#: In the below program, we use the do-while loop to calculate the factorial of a given number. The number here we are taking from the console.

using System; //calculating factorial with recursion namespace ConsoleApplication2 { class Program { long fun(long a) { if (a <= 1) { return 1;} else { long c = a * fun(a - 1); return c; }} static void Main(string[] args) { Console.WriteLine("enter the number"); long num = Convert.ToInt64(Console.ReadLine()); Console.WriteLine(new Program().fun(num)); Console.ReadLine(); } } }

**Write a C# program to calculate a factorial using recursion,** Factorial of a number is what we are finding using a recursive function checkFact () in the below example −If Login � Library � Videos � eBooks Write a C# program to calculate a factorial using recursion If not, then the recursive function will be called for the following iterations if you want the value of 5! Calculate the factorial of a given number: -------------------------------------------- Input the number : 6 The Factorial of 6 is: 720.

The following can calculate the factorial of 5000 in 1 second.

public class Number { #region Fields private static long _valueDivision = 1000000000; private static int _valueDivisionDigitCount = 9; private static string _formatZeros = "000000000"; List<long> _value; #endregion #region Properties public int ValueCount { get { return _value.Count; } } public long ValueAsLong { get { return long.Parse(ToString()); } set { SetValue(value.ToString()); } } #endregion #region Constructors public Number() { _value = new List<long>(); } public Number(long value) : this() { SetValue(value.ToString()); } public Number(string value) : this() { SetValue(value); } private Number(List<long> list) { _value = list; } #endregion #region Public Methods public void SetValue(string value) { _value.Clear(); bool finished = false; while (!finished) { if (value.Length > _valueDivisionDigitCount) { _value.Add(long.Parse(value.Substring(value.Length - _valueDivisionDigitCount))); value = value.Remove(value.Length - _valueDivisionDigitCount, _valueDivisionDigitCount); } else { _value.Add(long.Parse(value)); finished = true; } } } #endregion #region Static Methods public static Number operator +(Number c1, Number c2) { return Add(c1, c2); } public static Number operator *(Number c1, Number c2) { return Mul(c1, c2); } private static Number Add(Number value1, Number value2) { Number result = new Number(); int count = Math.Max(value1._value.Count, value2._value.Count); long reminder = 0; long firstValue, secondValue; for (int i = 0; i < count; i++) { firstValue = 0; secondValue = 0; if (value1._value.Count > i) { firstValue = value1._value[i]; } if (value2._value.Count > i) { secondValue = value2._value[i]; } reminder += firstValue + secondValue; result._value.Add(reminder % _valueDivision); reminder /= _valueDivision; } while (reminder > 0) { result._value.Add(reminder % _valueDivision); reminder /= _valueDivision; } return result; } private static Number Mul(Number value1, Number value2) { List<List<long>> values = new List<List<long>>(); for (int i = 0; i < value2._value.Count; i++) { values.Add(new List<long>()); long lastremain = 0; for (int j = 0; j < value1._value.Count; j++) { values[i].Add(((value1._value[j] * value2._value[i] + lastremain) % _valueDivision)); lastremain = ((value1._value[j] * value2._value[i] + lastremain) / _valueDivision); //result.Add((); } while (lastremain > 0) { values[i].Add((lastremain % _valueDivision)); lastremain /= _valueDivision; } } List<long> result = new List<long>(); for (int i = 0; i < values.Count; i++) { for (int j = 0; j < i; j++) { values[i].Insert(0, 0); } } int count = values.Select(list => list.Count).Max(); int index = 0; long lastRemain = 0; while (count > 0) { for (int i = 0; i < values.Count; i++) { if (values[i].Count > index) lastRemain += values[i][index]; } result.Add((lastRemain % _valueDivision)); lastRemain /= _valueDivision; count -= 1; index += 1; } while (lastRemain > 0) { result.Add((lastRemain % _valueDivision)); lastRemain /= _valueDivision; } return new Number(result); } #endregion #region Overriden Methods Of Object public override string ToString() { string result = string.Empty; for (int i = 0; i < _value.Count; i++) { result = _value[i].ToString(_formatZeros) + result; } return result.TrimStart('0'); } #endregion } class Program { static void Main(string[] args) { Number number1 = new Number(5000); DateTime dateTime = DateTime.Now; string s = Factorial(number1).ToString(); TimeSpan timeSpan = DateTime.Now - dateTime; long sum = s.Select(c => (long) (c - '0')).Sum(); } static Number Factorial(Number value) { if( value.ValueCount==1 && value.ValueAsLong==2) { return value; } return Factorial(new Number(value.ValueAsLong - 1)) * value; } }

**Calculate the Factorial of an Integer in C#,** Calculate the Factorial of an Integer in C# It could be done using Recursive Lambda Expression, for example,class Program{ static Func Factorial = x => x < 0 ? -1 : x == 1 || x == 0 ? 1 : x * Factorial(x - 1); static void Main(string[] args) { Console. WriteLine(Factorial(5)); }}(The code Finding the area of a trapezoid in C# with given sides a, b and the height h; How to find the last digit of a number "n" and print it in C#; Calculate Factorial of a number in JavaScript; Calculate the weight of a man on the Moon in C#; How to calculate the perimeter (circumference) of a circle in C#? How to calculate the area of a circle in C#?

**One line function for factorial of a number,** We can find the factorial of a number in one line with the help of Ternary operator or commonly known as Conditional operator in recursion. Recommended:� Let's solve factorial of number by using recursion. We know that in factorial number value is multiple by its previous number so our problem is divided in small part. using System; namespace FactorialExample { class Program { static void Main(string [] args) { Console.WriteLine("Enter a number");

**Factorial Function in Math Library,** Hey,I was wondering, does anybody know if c++ has a factorial function = in its math library? This code counts downwards if you start with a negative, and will run out of stack space and crash your application. My Casio calculator holds 100 decimal digits, which is good to 69! Error When Specifying Code in C#. Previous: Write a program in C# Sharp to Check whether a given String is Palindrome or not using recursion. Next: Write a program in C# Sharp to find the Fibonacci numbers for a n numbers of series using recursion.

**Math Class (System),** several mathematical and trigonometric functions from the Math class to calculate the inner C# Copy. Run. /// <summary> /// The following class represents simple functionality </summary> using System; namespace MathClassCS { class� Factorial is undefined for negative numbers. This code counts downwards if you start with a negative, and will run out of stack space and crash your application. Factorial grows rather fast. In a really old machine, int means a 16-bit signed int, and 7! is the highest which that will hold.

##### Comments

- See also the same in Math.NET Numerics, the successor of the linked Math.NET Iridium library.
- Checking it out now-- man, I really wish there were some best-approximation-may-not-work-for-everyone implementation as part of the base libraries.
- The problem with that code is that the downloads appear to be all in java, and I'm just not interested in the work to convert from java to C#. The author probably has C# code to download, but the Math.Net solution puts the result into a double, which is exactly what I need.
- And what happens when n ~= 50 and k ~= 2? Overflow! I need some way to handle nontrivial cases.
- In that case you calculate n = 50 and k = 48 as I pointed out.
- ok. What's 48!? Will that fit into an integer? Because that's what your denominator is doing in that case.
- Regardless of the algorithm, the result of, say, nCr for n = 100 and r = 50 will overflow a 64 bit number. You need to go back to your original problem because you can't store that result.
- Only if I don't go with an algorithm that uses something like Stirling's Approximation or the like, putting the result into a double. The next step in the algorithm is a rudimentary integration, so 'close enough' will be just fine, and the precision loss on a double is ok.
- Please read the other answers. This is not a sufficient solution, because long does not have the capacity to handle the size of a number near 100!. As such, some other method (such as Stirling's approximation) need to be used instead.
- It doesn't do run in 1 second on my computer ;)