Tip/Trick

Calculate the Factorial of an Integer in C#

mcjohnson

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3.33/5 (5 votes)

4 Oct 2011CPOL

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I looked for a while over the net for a way to easily calculate a factorial value (n!), but nothing was helping and I saw some VERY long and drawn out solutions. Well, it wasn't that hard after I thought about it for a while and this was the easiest solution I could come up with. I hope it helps!

The code below is what I put in my event handler. The comment lists my class variables used.

int Factorial(int input)
{
    int answer = 0;

    if (input > 0)
    {
        count = 1;
        while (count <= input)
        {
            if (count == 1)
            {
                answer= 1;
                count++;
            }
            else
            {
                answer = count * answer;
                count++;
            }
        }
    }
    else
    {
        MessageBox.Show("Please enter only a positive integer.");
    }

    return answer;
}

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Comments and Discussions

Linq Version

raulito17-Oct-16 5:30

raulito

17-Oct-16 5:30

Here is a Linq version that works like a charm...

List<int> retval=
    Factorial(
        5).ToList();

The list position zero will have the answer: 120.

private IEnumerable<int> Factorial(
      int n)
  {
      if (n > 1)
          return from b in Factorial(n - 1)
                 select n * b;
      else
          return new List<int>() { n };
  }

Check this one

Mathew Sachin24-Dec-14 4:26

Mathew Sachin

24-Dec-14 4:26

public static double Factorial(int n)
    {
        if (n < 0) return Double.NaN;
        else if (n == 0) return 1;
        else
        {
            double Product = 1;
            for (int i = 1; i <= n; ++i) Product *= i;
            return Product;
        }
    }

Mathew Sachin

Fail for higher order numbers

CyberSamuraiii13-May-14 7:21

CyberSamuraiii

13-May-14 7:21

This will not work for numbers greater than 16 unless you use long type or int64

Zero factorial is not undefined

Yitzi916-Aug-13 3:06

Yitzi9

16-Aug-13 3:06

Zero factorial is not undefined, it is equal to one.

What about?

int Factorial(int input)
{
    if (input < 0) throw new ArgumentOutOfRangeException("Factorial is only defined for non-negative integers.");
    int answer = 1;
    while (input > 1) answer *= input--;
    return answer;
}

Slight Improvement

markbb3322-Feb-13 18:36

markbb33

22-Feb-13 18:36

Your answer works, but can I suggest a small modification to capture an overflow exception. The following returns a negative number on error otherwise returns the result.

private static int factorial(int input) {
	if (input <= 0) return -1;
	try {
		int answer = 1;
		for (int count = 2; count <= input; count++) answer = checked(count * answer);
		return answer;
	}
	catch (OverflowException) {
		return -1;
	}
}