On 12/09/2011 03:04 PM, surya k wrote:
----------------------------------------
Date: Fri, 9 Dec 2011 14:53:07 -0500
From: d...@davea.name
To: sur...@live.com
CC: tutor@python.org
Subject: Re: [Tutor] how to handle big numbers
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
Well, its in a puzzle.. everything is done except this part. I need to
calculate N! ( max value of N is 10^6).
I decided to take the time, and the desired number is 1317440 digits
long, and takes about 3 seconds to compute. So try it, and forget my
stuff till you want a really large factorial.
I tried factorial of 10 times your number, and decided to kill it after
5 minutes. So if you have to go up to a million, my 3-part answer is
still appropriate.
If it's part of a larger puzzle, perhaps you are not really looking for
the actual value, but are going to do something with it. For example,
you may just want the last 1000 digits (I can tell you that answer
without a program).
What does your code look like so far? If it's more than one line,
perhaps you haven't loooked hard enough at the standard library. it has
both gamma and factorial functions. Unfortunately, the gamma function
runs out of steam at less than 180, when the float just isn't big enough.
There are other approaches that may bear looking at, like the SciPy
library. But first we'd need to know just what you need with this
factorial.
--
DaveA
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