Generic Factorial function in C#

Question

I want to write a generic function to calculate factorial in C# ... like:

 static T Factorial<T>(T n)
        {
            if (n <= 1)
                return 1;

            return Factorial<T>(n - 1);
        }

but obviously having restriction that we can't perform operations on type 'T'. any alternative?

Answer 1

The problem is that generics don't support operators because they are static methods, and not part of an interface. However, you could probably use Generic Operators, which is available in the Miscellaneous Utility Library.

Answer 2

There is no easy way to do this. I have seen some solutions that work around the problem, but they are fairly complicated. That said, if you really want to do this here are a few ideas:

1. If you can use .Net 4, you can cast n to dynamic and then perform the addition. You lose safety, of course - you could get an exception at runtime

2. You could always manually check the type from within your factorial function: If n is a short, cast to short, if n is a double, cast to double... etc. That is complicated and defeats part of the value of generics, but the outside API at least looks simple.

Answer 3

You would need to add a delegate parameter which performs the multiplication. Something like this

delegate T Multiply<T>(T a, T b);

So then your function would be defined like this:

static T Factorial<T>(T n, Multiply func)
{
    ... your code here
}

So when your factorial function is called, the caller would pass in the multiplication function:

int x = Factorial<int>(5, (a,b) => a * b);

Answer 4

When's the last time you took the factorial of a string, or a character? Why do you ever need a factorial of type T????

Besides this has been said numerous (prolly 1 million times now). When you need to use a generic you need to tell the compiler the type.

For instance what if I had a generic stack class? C# needs to know the elements type when I create my stack.

Otherwise it makes no sense for:

Stack<T> s = new Stack<T>();
s.Push(????); //how do I add items to the stack if we don't know what T is?

Instead you need to specify:

Stack<int> s = new Stack<int>();
s.Push(5);
s.Push(7);

Answer 5

This isn't specifically addressing your question about making the method generic, but your method as it stands will never return anything other than 1.

Supposing you were only working with integers, it should look like this:

static int Factorial(int n)
{
    if (n <= 1)
        return 1;

    // note the multiplication in this step
    return n * Factorial(n - 1);
}

Answer 6

 public T Factorial<T>(T a, T b, Multiply<T> delegateMutliply, Difference<T> diffDelegate, BaseCondition<T> baseDelegate)
    {
        if (!baseDelegate(a, b))
            return b;

        return delegateMutliply(a, Factorial<T>(diffDelegate(a), b, delegateMutliply, diffDelegate, baseDelegate));
    }

int y = p.Factorial(3, 1, (a, b) => a * b, (a) => --a, (a, b) => (a <= b) ? false : true);