Java::比较舍入的双精度浮点数不起作用。

huangapple
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英文:

Java :: comparing ronded doubles doesn't work

问题

我想比较两个数字：

  • -3.123
  • -3.123456

我将使用 double 来存储它们，我只想考虑前3位小数，所以：

  1. import java.math.RoundingMode;
  2. import java.text.DecimalFormat;
  4. public class DecimalComparator {
  5.     public static void main(String[] args) {
  6.         areEqualByThreeDecimalPlaces(-3.123, -3.123456);
  7.     }
  9.     public static boolean areEqualByThreeDecimalPlaces(double one, double two) {
  10.         boolean same = true;
  12.         DecimalFormat df = new DecimalFormat("#.###");
  13.         df.setRoundingMode(RoundingMode.FLOOR);
  14.         System.out.println(df.format(one));
  15.         System.out.println(df.format(two));
  17.         if (df.format(one).equals(df.format(two))) {
  18.             same = true;
  19.             System.out.println("true");
  20.         } else {
  21.             same = false;
  22.             System.out.println("false");
  23.         }
  24.         return same;
  25.     }
  26. }

这段代码返回给我：

  1. -3.123
  2. -3.124
  3. false

为什么第二个数字会四舍五入为 -3.124？

英文:

I want to compare two numbers:

  • -3.123
  • -3.123456

I will use double to store them and I just want to consider the first 3 decimals, so:

  1. import java.math.RoundingMode;
  2. import java.text.DecimalFormat;
  4. public class DecimalComparator {
  5.     public static void main(String[] args) {
  6.         areEqualByThreeDecimalPlaces(-3.123, -3.123456);
  7.     }
  9.     public static boolean areEqualByThreeDecimalPlaces (double one, double two) {
  10.         boolean same = true;
  12.         DecimalFormat df = new DecimalFormat("#.###");
  13.         df.setRoundingMode(RoundingMode.FLOOR);
  14.         System.out.println(df.format(one));
  15.         System.out.println(df.format(two));
  17.         if (df.format(one).equals(df.format(two))) {
  18.             same = true;
  19.             System.out.println("true");
  20.         } else {
  21.             same = false;
  22.             System.out.println("false");
  23.         }
  24.         return same;
  25.     }
  26. }

The code is returning me:

  1. -3.123
  2. -3.124
  3. false

why the second number is rounding to -3.124?

答案1

得分: 1

  1.     Hi its just that when u round off 3.123456 it works the below way when decimal
  2.     places are reduced :
  4.      3.12346
  5.      3.1235
  6.      3.124
  7.      3.12
  8.      3.1 
  10.     // commenting the set rounding mode will make it work the way u want.
  11.     public static void main (String[] args ) {
  12.         double one =-3.123;
  13.         double two = -3.123456;
  14.         boolean same = true;
  16.         DecimalFormat df = new DecimalFormat("#.###");
  17.         // remove this line
  18.         // df.setRoundingMode(RoundingMode.FLOOR);
  20.         System.out.println(df.format(one));
  21.         System.out.println(df.format(two));
  23.         if (df.format(one).equals(df.format(two))) {
  24.             same = true;
  25.             System.out.println("true");
  26.         } else {
  27.             same = false;
  28.             System.out.println("false");
  29.         }
  30.         System.out.println(same);
  31.     }
英文: 
  1. Hi its just that when u round off 3.123456 it works the below way when decimal 
  2. places are reduced :
  3.  
  4.  3.12346
  5.  3.1235
  6.  3.124
  7.  3.12
  8.  3.1 
  9.  
  10. //commenting the set rounding mode will make it work the way u want.
  11. public static void main (String[] args ) {
  12.         double one =-3.123;
  13.          double two = -3.123456;
  14.         boolean same = true;
  16.         DecimalFormat df = new DecimalFormat("#.###");
  17.         //remove this line
  18.         //df.setRoundingMode(RoundingMode.FLOOR);
  19.        
  20.         System.out.println(df.format(one));
  21.         System.out.println(df.format(two));
  23.         if (df.format(one).equals(df.format(two))) {
  24.             same = true;
  25.             System.out.println("true");
  26.         } else {
  27.             same = false;
  28.             System.out.println("false");
  29.         }
  30.         System.out.println(same);
  31.     }

答案2

得分: 0

RoundingMode.FLOOR会将数字向下舍入 - 您的代码对于正数可以工作，但对于负数则不行。您需要使用 RoundingMode.DOWN ，它只是在第 N 位之后删除数字：

  1. df.setRoundingMode(RoundingMode.DOWN);
  2. // 在这里 ------------------------^
英文:

RoundingMode.FLOOR rounds the number towards the lower side - your code would work for positive numbers, but not for negative numbers. You need to use RoundingMode.DOWN that just removes digits after N places:

  1. df.setRoundingMode(RoundingMode.DOWN);
  2. // Here ------------------------^

huangapple
  • 本文由 发表于 2020年4月10日 16:44:51
  • 转载请务必保留本文链接：https://go.coder-hub.com/61136849.html
  • boolean
  • compare
  • decimal
  • double
  • java
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