使用BigDecimal计算PI [英] Calculating PI with BigDecimal

问题描述

这是我第一次使用BigDecimal，但遇到了一些麻烦.

This is my first time using a BigDecimal and I am having some troubles with it.

这是我的代码:

import java.math.BigDecimal;

public class Test {
    public static void main(String[] args) {
        BigDecimal pi = new BigDecimal(4);
        boolean plus = false;
        for (int i = 3; i < 1000000; i += 2) {
            if (plus) {
                pi.add(new BigDecimal(4).divide(new BigDecimal(i), 2, BigDecimal.ROUND_HALF_UP));
            } else {
                pi.subtract(new BigDecimal(4).divide(new BigDecimal(i), 2, BigDecimal.ROUND_HALF_UP));
            }
            plus = !plus;
        }
        System.out.println(pi);
    }
}

为什么它没有像应该那样计算PI，却给了我4个答案?

Why doesn't it calculate PI like it should but instead it gives me and answer of 4?

推荐答案

您永远不会更改pi.请记住，BigDecimals是不可变的，并且您永远都不会更改自己的值，即，您永远不会将其分配给新值，因此您必须在某个地方

You never change pi. Remember that BigDecimals are immutable, and you never change yours, i.e., you never assign it to a new value, and so you have to have somewhere

pi = ...

在您的if/else里面是个不错的地方.

Inside your if/else would be a good place for this.

if (plus) {
    pi = pi.add(new BigDecimal(4).divide(new BigDecimal(i), 2, BigDecimal.ROUND_HALF_UP));
} else {
    pi = pi.subtract(new BigDecimal(4).divide(new BigDecimal(i), 2, BigDecimal.ROUND_HALF_UP));
}

从 BigDecimal API的第一行起

不变的，任意精度的带符号十进制数字.

Immutable, arbitrary-precision signed decimal numbers.

我还将将2的比例更改为更好的水平.

I would also change that scale of 2 to something better.

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