Calculate Pi on an Android phone

Question

I'm trying to make an Android phone calculate n decimals of Pi. For this I'm using this code:

public class Pi {

private static final BigDecimal TWO = new BigDecimal(2);
private static final BigDecimal FOUR = new BigDecimal(4);
private String nummer; 

public void pi(final int SCALE) {
    BigDecimal a = ONE;
    BigDecimal b = ONE.divide(sqrt(TWO, SCALE), SCALE, ROUND_HALF_UP);
    BigDecimal t = new BigDecimal(0.25);
    BigDecimal x = ONE;
    BigDecimal y;

    while (!a.equals(b)) {
        y = a;
        a = a.add(b).divide(TWO, SCALE, ROUND_HALF_UP);
        b = sqrt(b.multiply(y), SCALE);
        t = t.subtract(x.multiply(y.subtract(a).multiply(y.subtract(a))));
        x = x.multiply(TWO);
    }

    a.add(b).multiply(a.add(b)).divide(t.multiply(FOUR), SCALE, ROUND_HALF_UP);
    number(a);
}

public void number(BigDecimal a) {
    //nummer = a.toString().length();
    nummer = a.toString();
}

public String returnNum() {
    return nummer;
}

public static BigDecimal sqrt(BigDecimal A, final int SCALE) {
    BigDecimal x0 = new BigDecimal("0");
    BigDecimal x1 = new BigDecimal(Math.sqrt(A.doubleValue()));

    while (!x0.equals(x1)) {
        x0 = x1;
        x1 = A.divide(x0, SCALE, ROUND_HALF_UP);
        x1 = x1.add(x0);
        x1 = x1.divide(TWO, SCALE, ROUND_HALF_UP);
    }

    return x1;
}

This works like a charm on any computer but when I try to run this on my Android tablet it decides that Pi is 0.8 something. Why does this miss calculation occur and how can I make this work?

Answer 1

You are missing a bit from your calculation. The line :

a.add(b).multiply(a.add(b)).divide(t.multiply(FOUR), SCALE, ROUND_HALF_UP);

is not being assigned to anything. Assign this to something and print that value out and you'll find your calculation works.

BigDecimal pi = a.add(b).multiply(a.add(b)).divide(t.multiply(FOUR), SCALE, ROUND_HALF_UP);
System.out.println(pi)

This gives me the correct answer.

For those that are interested, the algorithm can be found on wikipedia