D为什么0.1 + 0.2 == 0.3？ [英] Why 0.1 + 0.2 == 0.3 in D?

问题描述

  assert（0.1 + 0.2！= 0.3）; //应该是真的
  

是我最喜欢的一个语言使用本地浮点算法的检查。 >

C ++

  #include< cstdio> 
 
 int main（）
 {
 printf（％d \ n，（0.1 + 0.2！= 0.3））; 
返回0; 
 
 
 
 $ b $ p $输出：
 
 
  1 
  
  http://ideone.com/ErBMd  
 
 $ b 
 Python 
 
 
 
  print（0.1 + 0.2！= 0.3）
  
输出：
 
 
  True 
  
  http://ideone.com/TuKsd  
 
 
 
其他示例
 
 
 
 
 
 Java： http://ideone.com/EPO6X  
 
 
 C＃： http://ideone.com/s14tV  
 
 
 
 
 
 D为什么不是这样？理解D使用本地浮点数。这是一个错误？他们是否使用一些具体的数字表示？别的东西？  
 
 
 D 
 
 
 
  import std.stdio; 
 
 void main（）
 {
 writeln（0.1 + 0.2！= 0.3）; 
 
 
 
 $ b $ p $输出：
 
 
  false 
  
  http://ideone.com/mX6zF   
 
 
 UPDATE 
 
 
 
感谢 LukeH 。这是浮点常量折叠的效果，描述如下 。 
 
 
 
代码： 
 
 
  import std.stdio; 
 
 void main（）
 {
 writeln（0.1 + 0.2！= 0.3）; //常量折叠是以真实精度完成
 
 auto a = 0.1; 
 auto b = 0.2; 
 writeln（a + b！= 0.3）; //以双精度计算的标准计算
 
 code $ 
 $ p $输出：
 
 
  false 
 true 
  
  http://ideone.com/z6Zlk  
 
assert(0.1 + 0.2 != 0.3); // shall be true
is my favorite check that a language uses native floating point arithmetic.

C++

#include <cstdio>

int main()
{
   printf("%d\n", (0.1 + 0.2 != 0.3));
   return 0;
}
Output:
1
http://ideone.com/ErBMd

Python

print(0.1 + 0.2 != 0.3)
Output:
True
http://ideone.com/TuKsd

Other examples


Java: http://ideone.com/EPO6X
C#: http://ideone.com/s14tV


Why is this not true for D? As understand D uses native floating point numbers. Is this a bug? Do they use some specific number representation? Something else? Pretty confusing.

D

import std.stdio;

void main()
{
   writeln(0.1 + 0.2 != 0.3);
}
Output:
false
http://ideone.com/mX6zF

UPDATE

Thanks to LukeH. This is an effect of Floating Point Constant Folding described there. 

Code:
import std.stdio;

void main()
{
   writeln(0.1 + 0.2 != 0.3); // constant folding is done in real precision

   auto a = 0.1;
   auto b = 0.2;
   writeln(a + b != 0.3);     // standard calculation in double precision
}
Output:
false
true
http://ideone.com/z6ZLk
解决方案
It's probably being optimized to (0.3 != 0.3). Which is obviously false. Check optimization settings, make sure they're switched off, and try again.

                        
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