即使使用MathContext，也会出现非终止小数错误

Question

我正在制作代码来实现这个算法：

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然而，我得到了这个错误，即使使用MathContext(1000)也是如此：

Exception in thread "main" java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.
at java.math.BigDecimal.divide(BigDecimal.java:1603)
at picalculator.PiCalculator.calculatePi(PiCalculator.java:59)
at picalculator.PiCalculator.main(PiCalculator.java:25)
Java Result: 1

使用此方法时：

public static void calculatePi() {
    BigInteger firstFactorial;
    BigInteger secondFactorial;
    BigInteger firstMultiplication;
    BigInteger firstExponent;
    BigInteger secondExponent;
    int firstNumber = 1103;
    BigInteger firstAddition;
    BigDecimal currentPi = BigDecimal.ONE;
    BigDecimal pi = BigDecimal.ONE;
    BigDecimal one = BigDecimal.ONE;
    int secondNumber = 2;
    double thirdNumber = Math.sqrt(2.0);
    int fourthNumber = 9801;
    BigDecimal prefix = BigDecimal.ONE;

    for(int i=1;i<4;i++){
        firstFactorial = factorial(4*i);
        secondFactorial = factorial(i);
        firstMultiplication = BigInteger.valueOf(26390*i);
        firstExponent = exponent(secondFactorial, 4);
        secondExponent = exponent(BigInteger.valueOf(396),4*i);
        firstAddition = BigInteger.valueOf(firstNumber).add(firstMultiplication);
        currentPi = currentPi.add(new BigDecimal(firstFactorial.multiply(firstAddition)).divide(new BigDecimal(firstExponent.multiply(secondExponent)), new MathContext(10000)));
    }

    prefix =new BigDecimal(secondNumber*thirdNumber);
    prefix = prefix.divide(new BigDecimal(fourthNumber), new MathContext(1000));

    currentPi = currentPi.multiply(prefix, new MathContext(1000));

    pi = one.divide(currentPi);

    System.out.println("Pi is: " + pi);

    return;
}

我已经证明，阶乘(A)；和指数(a，b)分别准确地返回a的阶乘和a^b的结果。

有人知道怎么解决这个问题吗？

Answer 1

你需要

pi = one.divide(currentPi, new MathContext(1000));

因为结果几乎肯定是一个重复的小数。

考虑一下

BigDecimal a = new BigDecimal("4");
BigDecimal b = new BigDecimal("3");

BigDecimal c = a.divide(b)                         // java.lang.ArithmeticException: Non-terminating decimal expansion
BigDecimal c = a.divide(b, new MathContext(10));   // No exception

Answer 2

您可能更喜欢使用不同版本的divide。它使您能够更好地控制返回的BigDecimal的最终规模。然而，对于您的版本，最终的比例取决于被除数和除数的比例。

int scale = 3;
BigDecimal result = ONE.divide(new BigDecimal("3"), scale, RoundingMode.HALF_UP);
// result is 0.333