Chudnovsky algorithm

Question

I tried to make a program (in Java) that calculates pi with the Chudnovsky algorithm but it has the output NaN (Not a Number). Please help me find mistakes in my code, or improve my code. (I don't have a lot of Java programming knowledge)

You can find Chudnovsky's algorithm here:

https://en.wikipedia.org/wiki/Chudnovsky_algorithm

here is my code:

package main;

public class Class1 {

    public static void main(String[] args)
    {
        double nr1=0,nr2=0,nr3=0,pi=0;
        int fo1=1, fo2=1, fo3=1;

        for(int i=0; i<=20; i++){

            for(int fl1=1; fl1<=(6*i); fl1++){fo1 = fo1 * fl1;}
            for(int fl2=1; fl2<=(3*i); fl2++){fo2 = fo2 * fl2;}
            for(int fl3=1; fl3<=(i); fl3++){fo3 = fo3 * fl3;}

            nr1 = ( (Math.pow(-1, i)) * (fo1) * ((545140134*i) + 13591409) );
            nr2 = ( (fo2) * (Math.pow(fo3, i)) * ( Math.pow(Math.pow(640320, 3), (i+(1/2)) )) );
            nr3 = 12 * (nr1/nr2);

        }

        pi = 1/nr3;
        System.out.println((Math.PI));
        System.out.println(pi);
    }
}

`i+(1/2)` probably doesn't do what you think. It doesn't evaluate to `i+0.5`, it evaluates to `i`. — Andy Turner, Mar 17 '16 at 12:04
Also, I doubt that `int` is an appropriate type for `fo1`, `fo2`, `fo3`. You appear to be calculating factorials like 120! which is way bigger than `Integer.MAX_VALUE`. — Andy Turner, Mar 17 '16 at 12:08

score 3 · Answer 1 · edited May 23 '17 at 12:00

There are many issues here.

As Andy mentioned, 1/2 is not 0.5.
You are using integers to compute things like 120! which is completely out of bounds for any primitive type.
f01,f02,f03 should be initialized inside each loop, otherwise they grow even bigger

It is not trivial to fix it. You can take a look at Error calculating pi using the Chudnovsky algorithm - Java and http://www.craig-wood.com/nick/articles/pi-chudnovsky/ for some hints, but don't expect built-in primitive types to work with that algorithm.

Chudnovsky algorithm

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