[Tutor] how to handle big numbers

[Dave Angel]

On 12/09/2011 02:25 PM, surya k wrote:
> Finding factorial of 8 or 9 isn't big. If I would like to find factorial of 32327, how can I ?? 		 	   		
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How close do you want your answer?  The gamma function can be used to 
calculate it, but it won't be precise.   I don't know how many digits 
are in 32327!, but it must be at least hundreds of thousands, and a 
float cannot represent that exactly.  In fact it cannot represent a 
number that big, even approximately.  It might be millions of digits, 
but I don't have the time right now to figure it out.

If you use a long int (which int will promote to, automatically), you 
could do the calculation with a very simple program, providing you don't 
run out of either time or memory.

if it were my problem, I'd do it in three steps.  First write a program 
to calculate N! exactly.  See how long it takes for 100, and how many 
digits are in the answer.  Then try it again for 1000!

Next, I'd look up the gamma function, and figure out how large the 
desired value will be.

Finally, depending on what I got from those first two, I'd either run 
the first program with 32327, and wait a long time, or write a 
specialized math package to calculate the gamma function to whatever 
precision I thought Ineeded.


-- 

DaveA