CODEWITHSUNDEEP
javabiginteger
jason adams
jason adams

Reputation: 565

How to use BigInteger class?

I am having trouble printing numbers past 12!(factorial).
Can someone please help me out? I'm not even sure if I'm using this class correctly. 

public class Application{

   public static int factorial(int n){

      int index = n;
      int total = 1;
      while(index > 0){
         total *= index;
         index --;
      }

      return total;
   }

   public static void print(int n){
      int index = n;
      while(index > 0){
         BigInteger big = BigInteger.valueOf(factorial(index));
         System.out.println(index + ": " + big);
         index --;
      }
   }

   public static void main(String[] args){
      int n = 30;
      print(n);

   }
}

and this is a snippet of what it prints out:

18: -898433024
17: -288522240
16: 2004189184
15: 2004310016
14: 1278945280
13: 1932053504
12: 479001600

Upvotes: 3

Views: 1592

Answers (2)

maerics
maerics

Reputation: 156434

The problem is that you are trying to do "big" math with plain integers (declared as int, using the multiplication operator *) and then wrapping in a BigInteger when you should instead be doing math on BigIntegers directly (e.g. using the BigInteger#multiply(...) method:

public static void main(String[] args) {
  BigInteger factorial = BigInteger.ONE;
  for (int i=1; i<=20; i++) {
    factorial = factorial.multiply(BigInteger.valueOf(i));
    System.out.println(i + "! = " + factorial);
  }
}

// 1! = 1
// 2! = 2
// 3! = 6
// ...
// 18! = 6402373705728000
// 19! = 121645100408832000
// 20! = 2432902008176640000

Upvotes: 2

durron597
durron597

Reputation: 32323

Use BigInteger in your factorial function, not "after you've done the calculation".

Note also that when multiplying BigIntegers, use the BigInteger.multiply(BigInteger val) method instead of *.

Here's the changed method, which is exactly the same as yours except it uses BigInteger instead of int:

public static BigInteger factorial(int n){
  int index = n;
  BigInteger total = BigInteger.valueOf(1);
  while(index > 0){
     total = total.multiply(BigInteger.valueOf(index));
     index --;
  }

  return total;
}

Note that also you don't need to convert the return value of the method to a BigInteger anymore, e.g. just do:

BigInteger big = factorial(index);

Here's the output:

30: 265252859812191058636308480000000
29: 8841761993739701954543616000000
28: 304888344611713860501504000000
27: 10888869450418352160768000000
26: 403291461126605635584000000
25: 15511210043330985984000000
24: 620448401733239439360000
23: 25852016738884976640000
22: 1124000727777607680000
21: 51090942171709440000
20: 2432902008176640000
19: 121645100408832000
18: 6402373705728000
17: 355687428096000
16: 20922789888000
15: 1307674368000
14: 87178291200
13: 6227020800
12: 479001600
11: 39916800
10: 3628800
9: 362880
8: 40320
7: 5040
6: 720
5: 120
4: 24
3: 6
2: 2
1: 1

Upvotes: 6

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