B# .NET Blog

One of the new features in .NET Framework 3.5 is the new namespace System.Numeric that lives in System.Core.dll. It's a pretty lonesome place for a class to be defined in: System.Numeric's only citizen at this very moment is BigInteger. But how happy it is to be there :-) A little tale...

Everyone knows the mathematical ! operator for factorials. Recursively it is defined as a unary operator on natural numbers:

0! = 1
n! = n * (n-1)! for n > 0

which can be implemented using a simple loop:

int res = 1;
for (int i = 2; i <= n; i++)
   res *= i;
return res;

The bottleneck? Take a look at the type of res, which is int (aka System.Int32). Further than 12! you won't get without overflowing. Replacing the type by long instead of int won't help us much further: after calculating 20! we're out of luck (question: would uint or ulong bring us any further?). Luckily BigInteger is here to help us out. A sample:

 1 using System;
 2 using System.Numeric;
 3 
 4 namespace ConsoleApplication1
 5 {
 6     class Program
 7     {
 8         static void Main(string[] args)
 9         {
10             FactorialWithInt32();
11             FactorialWithInt64();
12             FactorialWithBigInteger();
13         }
14 
15         static void FactorialWithInt32()
16         {
17             int res = 1;
18 
19             try
20             {
21                 checked
22                 {
23                     for (int n = 2; ; n++)
24                         Console.WriteLine("{0}! = {1}", n, res *= n);
25                 }
26             }
27             catch { }
28         }
29 
30         static void FactorialWithInt64()
31         {
32             long res = 1;
33 
34             try
35             {
36                 checked
37                 {
38                     for (int n = 2; ; n++)
39                         Console.WriteLine("{0}! = {1}", n, res *= n);
40                 }
41             }
42             catch { }
43         }
44 
45         static void FactorialWithBigInteger()
46         {
47             BigInteger res = 1;
48             
49             try
50             {
51                 checked
52                 {
53                     for (int n = 2; ; n++)
54                         Console.WriteLine("{0}! = {1}", n, res *= n);
55                 }
56             }
57             catch { }
58         }
59     }
60 }

The checked keyword performs overflowing checks and is needless in the FactorialWithBigInteger sample; BigInteger won't run out of juice pretty soon (basically, a BigInteger is an abstraction of an array of integer values together with a sign, with overloads for typical integer operators). So, if you ever need to deal with large (I mean really large!) integers, BigInteger might be something for you. However, keep in mind that BigIntegers are slower in use (after all, it has to deal with much more data than a simple add operation) so unless you really need to keep a large value, stay away from it.

A few notes

What do I need to run the sample? Answer: download the January 07 CTP of Orcas

Where is System.Core.dll? Answer: take a look at %windir%\Microsoft.NET\Framework\v3.5.11209