Overflow exception? | 易学教程

问题

I have the following code for finding factorials:

Private Shared Function Factorial(ByVal Number As Long) As Long
        If Number = 0 Then
            Return 1
        Else
            Return Number * Factorial(Number - 1)
        End If
End Function

It usually results in an overflow. It only works if I start with something small like 4.

I have to work with starting numbers such as 30-60.

Any ideas? I thought changing the value type to LONG would prevent this problem.

This is VB.net just for reference.

回答1:

Factorials get very large, very quickly. The largest number that will fit in a Long is about 9×10^18. Factorial(30) is about 2.7×10^32.

If you're using .Net 4 there is a built-in BigInteger class that you can use which will hold arbitrarily large numbers.

If you're not using .Net 4, you'll need to find and download a BigInteger library, for example intx.

回答2:

You get the overflow exception only with integer and long types. To avoid that, you can use System.Double or System.Numerics.BigInteger (or BigDecimal I think).

For example, if you run 3 different versions of the factorial: 1 with long, 1 with double and 1 with biginteger as follow with a range of values of 5 to 50 by 5:

    'Long Factorial   
    Public Function FactorialInt64(ByVal n As Integer) As Int64  
        If n = 1 Then  
            Return 1  
        Else  
            Return n * FactorialInt64(n - 1)  
        End If  
    End Function  
    ' Double Factorial   
    Public Function FactorialDouble(ByVal n As Integer) As Double  
        If n = 1 Then  
            Return 1  
        Else  
            Return n * FactorialDouble(n - 1)  
        End If  
    End Function  
    ' BigInteger Factorial   
    Public Function FactorialBigInteger(ByVal n As Integer) As BigInteger  
        If n = 1 Then  
            Return 1  
        Else  
            Return n * FactorialBigInteger(n - 1)  
        End If  
    End Function

You will get a result like this:

You can find the complete source code in my blog post: Factorial and Fibonacci in VB.NET

回答3:

There is a big int Library for .NET that will solve your problem. It can manipulate very large number (limited only by your system memory).

Here is the link: http://www.emilstefanov.net/Projects/GnuMpDotNet/

来源：https://stackoverflow.com/questions/5483940/overflow-exception

标签

vb.net

overflow

factorial