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How to compute large factorial values

Posted on 2015-02-21
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Last Modified: 2015-03-01
I am trying to compute a value for 80! (80 factorial) in Java. I have tried both long and BigInteger types without success. I have seen where Guava has a BigIntegerMath class but, unfortunately, I am not allowed to load those jar files. Is there any I can accomplish this without the BigIntegerMath class? If so, can an example be provided?
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Question by:awking00
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by:ste5an
ID: 40623145
Without success is not an information we can work with. Especially as factorials with BigInteger works.
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by:frankhelk
ID: 40623333
I think "Without success" is another word for "Overflow". 80! is a fairly big number and would bust the long type easily.

You could try with double (and loose precision).

I'fe googled a bit and found out that the Java platform (SE7) supports BigInteger ... so it should be usable to you w/o any external jar (see here for info about that class).

Maybe the class has no overloaded opertors, so you have to use methods to do calculations - i.e. (syntax not checked, more concept ...)

BigInteger a;
BigInteger b;
BigInteger product;

a = someValue;
b = anotherValue;
product = a.multiply(b);

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dpearson earned 250 total points
ID: 40623803
I'm not sure what problem you're having with BigInteger.  It should be exactly what you need.
Something along these lines:

		BigInteger fac = new BigInteger("1") ;

		for (int i = 2 ; i <= 80 ; i++) {
			BigInteger multiplier = new BigInteger(Integer.toString(i)) ;
			fac = fac.multiply(multiplier) ;
		}

		System.out.println("Factorial 80 is " + fac) ;

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by:mccarl
mccarl earned 250 total points
ID: 40625061
Just wanted to point out an alternative to Doug's solution (don't award any points to my comment), it wouldn't be much different in terms of efficiency but just feels better to not convert to strings...
		BigInteger fac = BigInteger.ONE;

		for (int i = 2 ; i <= 80 ; i++) {
			BigInteger multiplier = BigInteger.valueOf(i);
			fac = fac.multiply(multiplier);
		}

		System.out.println("Factorial 80 is " + fac);

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by:dpearson
ID: 40625101
Oh yeah - mccarl's approach is cleaner.

I'm just used to only using BigInteger when I have a value that needs to be represented as a String so I'm always using the String constructor - but of course that's not necessary here :)

Doug
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by:mccarl
ID: 40638634
Sorry Doug,

My intention was not to steal points off you, I even asked for no points! :)
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by:dpearson
ID: 40638732
Haha - no problem mccarl.

Personally I'm none too concerned about the points on EE - I think the goal of just helping people get good advice should be enough and you made this advice better :)

Doug
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