问Math.pow奇异结果
EN

Stack Overflow用户

提问于 2014-02-02 14:00:09

回答 3查看 538关注 0票数 0

我正在处理一个抵押贷款计算公式，并且得到了与Math.pow()不同的结果，我不知道为什么。

下面是测试设置：

double interestRatePercent = 7;
double monthlyInterestRate = (interestRatePercent / 100) / MONTHS_PER_YEAR;
int numberOfPayments = loanTermInYears * MONTHS_PER_YEAR;

Log.i(TAG, String.format("monthlyInterestRate: %f", monthlyInterestRate));
Log.i(TAG, String.format("numberOfPayments: %d", numberOfPayments));
Log.i(TAG, "  ");

Log.i(TAG, "Hardcoded result:");
double hardcodedResult = Math.pow(1.0 + 0.005833, numberOfPayments);
Log.i(TAG, String.format("(1 + 0.005833)^360 = %f", hardcodedResult));
Log.i(TAG, "  ");

Log.i(TAG, "Parameterized result:");
double paramResult = Math.pow(1.0 + monthlyInterestRate, numberOfPayments);
Log.i(TAG, String.format("(1 + %f)^%d = %f", monthlyInterestRate, numberOfPayments, paramResult));
Log.i(TAG, "  ");

Log.i(TAG, "BigDecimal result:");
BigDecimal bigResult = BigDecimal.valueOf(1.0 + monthlyInterestRate);
bigResult = bigResult.pow(numberOfPayments);
Log.i(TAG, String.format("(1 + %f)^%d = %f", monthlyInterestRate, numberOfPayments, bigResult));
Log.i(TAG, "  ");
Log.i(TAG, "  ");

以下是测试结果：

monthlyInterestRate: 0.005833
numberOfPayments: 360

Hardcoded result:
(1 + 0.005833)^360 = 8.115529

Parameterized result:
(1 + 0.005833)^360 = 8.116497

BigDecimal result:
(1 + 0.005833)^360 = 8.116497

只有硬编码的结果是正确的。Math.pow和BigDecimal.pow的结果都很糟糕。

有什么想法吗？

android

math

floating-point

ieee

java

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回答 3

Stack Overflow用户

回答已采纳

发布于 2014-02-02 14:08:02

BigDecimal bigResult = BigDecimal.valueOf(1.0 + monthlyInterestRate);

当您创建BigDecimal时，您将失去精度，因为1.0 + monthlyInterestRate将失去精度。要获得更高的精度，请为1.0创建一个BigDecimal，然后使用add()方法将其添加到值为0.005833的另一个BigDecimal中。然后使用BigDecimal pow()函数。

换句话说，从更简单的BigDecimal类型(而不是int或double类型)构建int。

票数 4

Stack Overflow用户

发布于 2014-02-02 14:05:18

7/100/12大约是0.00583333333，而不是0.005833。使用%f时，默认情况下，在小数点之后只看到6位数字。

票数 6

Stack Overflow用户

发布于 2014-02-02 14:19:17

这一差异是由于0.005833333333333333599324266316443754476495087146759033203125，舍入了monthlyInterestRate的实际值，在硬编码结果计算中为0.005833。为了方便起见，我修改了程序以使用System.out.println。我更改了monthlyInterestRate的输出以准确地打印它，然后在硬编码计算中使用该值。产出如下：

monthlyInterestRate: 0.005833333333333333599324266316443754476495087146759033203125
numberOfPayments: 360

Hardcoded result:
(1 + 0.005833333333333333599324266316443754476495087146759033203125)^360 = 8.116497

Parameterized result:
(1 + 0.005833)^360 = 8.116497

BigDecimal result:
(1 + 0.005833)^360 = 8.116497

以下是修改后的程序：

import java.math.BigDecimal;

public class Test {
    public static void main(String[] args) {
        double interestRatePercent = 7;
        int MONTHS_PER_YEAR = 12;
        int loanTermInYears = 30;
        double monthlyInterestRate = (interestRatePercent / 100)
                / MONTHS_PER_YEAR;
        int numberOfPayments = loanTermInYears * MONTHS_PER_YEAR;

        // System.out.println(String.format("monthlyInterestRate: %f",
        // monthlyInterestRate));
        System.out.println("monthlyInterestRate: "
                + new BigDecimal(monthlyInterestRate));
        System.out.println(String.format("numberOfPayments: %d",
                numberOfPayments));
        System.out.println("  ");

        System.out.println("Hardcoded result:");
        double hardcodedResult = Math
                .pow(1.0 + 0.005833333333333333599324266316443754476495087146759033203125,
                        numberOfPayments);
        System.out
                .println(String
                        .format("(1 + 0.005833333333333333599324266316443754476495087146759033203125)^360 = %f",
                                hardcodedResult));
        System.out.println("  ");

        System.out.println("Parameterized result:");
        double paramResult = Math.pow(1.0 + monthlyInterestRate,
                numberOfPayments);
        System.out.println(String.format("(1 + %f)^%d = %f",
                monthlyInterestRate, numberOfPayments, paramResult));
        System.out.println("  ");

        System.out.println("BigDecimal result:");
        BigDecimal bigResult = BigDecimal.valueOf(1.0 + monthlyInterestRate);
        bigResult = bigResult.pow(numberOfPayments);
        System.out.println(String.format("(1 + %f)^%d = %f",
                monthlyInterestRate, numberOfPayments, bigResult));
        System.out.println("  ");
        System.out.println("  ");
    }
}