help with factorial recursive

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help with factorial recursive

 
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  #1
Dec 30th, 2008
C++ Syntax (Toggle Plain Text) 
  1. #include<iostream>
  2. #include<fstream>
  3. #include<cmath>
  4. #include<iomanip>
  5.  
  6. using namespace std;
  7. unsigned __int64 Fact(unsigned __int64 x );
  8.  
  9.  
  10. int main(){
  11.  
  12. 	/*n! means n  (n  1)  ...  3  2  1
  13.  
  14. Find the sum of the digits in the number 100!*/
  15.  
  16.  
  17. cout<<Fact(100);
  18.  
  19.  
  20. }
  21. unsigned __int64 Fact(unsigned __int64 x )
  22. {
  23. 	__int64 num(0);
  24.  
  25. 	if(x==0)
  26. 		return 1;
  27. 	else
  28. 	{
  29. 	 num = x * Fact(x-1);
  30. 	 cout<<num<<"  "<<x<<endl;
  31. 	}
  32.  
  33. 	return num;
  34. }


my recursive function works. But only for small x. (ex. 5! =120).

but when i try (100!) it runs out of space and give NULL;

so can you hint on how i can fix the code so that the computer will compute 100factorial?
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Re: help with factorial recursive

 
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  #2
Dec 30th, 2008
OK here is a little improved version of it. Now it finds at maximum of
65!. Any hints on how to find 100!
C++ Syntax (Toggle Plain Text) 
  1. #include<iostream>
  2. #include<fstream>
  3. #include<cmath>
  4. #include<iomanip>
  5.  
  6. using namespace std;
  7. unsigned __int64 Fact(unsigned __int64 x );
  8.  
  9.  
  10. int main(){
  11.  
  12. 	/*n! means n  (n  1)  ...  3  2  1
  13.  
  14. Find the sum of the digits in the number 100!*/
  15.  
  16. //unsigned __int64 * fact = new unsigned __int64 (10000);
  17.  
  18. Fact(100);
  19.  
  20. }
  21. unsigned __int64 Fact(unsigned __int64 x )
  22. {
  23. 	unsigned __int64 * fact = new unsigned __int64 (10000);
  24. 	*fact = x;
  25.  
  26. 	if(x==0)
  27. 		return 1;
  28. 	else
  29. 	{
  30. 	 *fact = x * Fact(x-1);
  31. 	 cout<<*fact<<"  "<<x<<endl;
  32. 	}
  33.  
  34. 	return *fact;
  35. 	delete fact;
  36. }
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Re: help with factorial recursive

 
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  #3
Dec 30th, 2008
Can't you use a loop instead?

You could create a class that grows and manages the value split over vectors, overloads the general math operators you'd need, then stores the values into a file.
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Re: help with factorial recursive

 
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  #4
Dec 30th, 2008
checked again anything over 20! is not correct. anyhelp?
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Re: help with factorial recursive

 
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  #5
Dec 31st, 2008
Check out GMP (Gnu MultiPrecision). It allows you to work with extremely large numbers and has wrappers for c++.
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Re: help with factorial recursive

 
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  #6
Dec 31st, 2008
To give you a clue about what's happening, 100! is about 9.3 by 10^157. To represent that, a 525 bit integer is needed (and that increases 530 bits to represent 101!). 64 bit is nowhere near enough: an unsigned 64 bit integer can only represent a maximum value of about 18446744073709551615 (approx 1.8 by 10^19).
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