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Dec 30th, 2008
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help with factorial recursive

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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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firstPerson is offline Offline
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Dec 30th, 2008
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Re: help with factorial recursive

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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firstPerson is offline Offline
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Dec 30th, 2008
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Re: help with factorial recursive

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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MosaicFuneral is offline Offline
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Re: help with factorial recursive

checked again anything over 20! is not correct. anyhelp?
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firstPerson is offline Offline
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Dec 31st, 2008
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Re: help with factorial recursive

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

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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grumpier is offline Offline
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This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.
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