指定无穷大来浮动 [英] Assign infinity to float

问题描述

  float f [] =>我想要初始化一个float值为infinity，而不使用INFINITY宏。 {
 0b01111111100000000000000000000000，// 0x7f800000 
 0x7f800000 -1，
 0x7f800000 -2，
 0x7f800000 -64，
 0x7f800000 -65 
}; 
  

印制：

  2139095040.000000 
 2139095040.000000 
 2139095040.000000 
 2139095040.000000 
 2139094912.000000 
  

为什么不是第一个数据无穷大（像 1 / 0.0 ），而其他数据只有在最后一个数据才会改变？

0x7f7fffff应该是浮点数最大的值，然后尝试这里 0x7f80000被认为是无穷大。

解决方案

您的问题的原因是符号：

• 0x7f800000


• 0b01111111100000000000000000000000 < code
  
  
  

  这个符号与 int 类型有关，当你将 int 分配给 float ，这意味着隐式转换 from int 到 float 。在这种情况下，你的两个数字都是十进制的 2139095040 ，它将被定位为 float type。

  



  为了避免这个问题，你可以在4个字节的精确位位置上赋值。这里有几个例子。
  
  

    float f; 
   *（int *）& f = 0x7f800000; 
    

  或者您可以使用工会

    union u_fi {
   float f; 
   int i; 
  } fi; 
   fi.i = 0x7f800000; 
    

  但使用这2种解决方案时要小心。如果 int 在第一种情况下超过4个字节，并且在 int 是 big-endian 。所以这个解决方案是平台相关的，我推荐使用如下所示的宏。

  



  你的问题的另一个解决方案是使用非常大的数字转换为 float 为 inf 。为此，您可以使用< math.h>

    #define _HUGE_ENUF 1e + 300 
   #define INFINITY（（float）（_ HUGE_ENUF * _HUGE_ENUF））
   float f = INFINITY; 
    

  
  

  I'm trying to initialize a float to value infinity, and without using INFINITY macro.

  float f[] = {
              0b01111111100000000000000000000000, // 0x7f800000
              0x7f800000 -1,
              0x7f800000 -2,
              0x7f800000 -64,
              0x7f800000 -65
          };
  

  Printed:

  2139095040.000000
  2139095040.000000
  2139095040.000000
  2139095040.000000
  2139094912.000000
  

  Why isn't the first data infinity (like 1 / 0.0) and the others don't change until the last data?

  0x7f7fffff should be the biggest value that a float can be and trying here 0x7f80000 is considered infinity.

  解决方案

  The reason of your problem is notations:

  • 0x7f800000
  • 0b01111111100000000000000000000000

  This notations related to int type and when you assigning int to float it means implicit conversion from int to float. In this case both of your numbers is 2139095040 in decimal and it will be conerted to float type.

  To avoid this problem you may assign value in exact bit positions of 4 bytes. Here a couple of examples.

  float f;
  *(int*)&f = 0x7f800000;
  

  Or you can use unions

  union u_fi {
      float f;
      int   i;
  } fi;
  fi.i = 0x7f800000;
  

  But be careful when use this 2 solutions. It won't work safely when int is more than 4 bytes in first case and won't work at all when int is big-endian. So this solutions are platform dependent and I recommend to use macro like it shown below.

  Another solution to your problem is to use very large number wich converts to float as inf. For this you can use macro as in <math.h>

  #define _HUGE_ENUF  1e+300
  #define INFINITY   ((float)(_HUGE_ENUF * _HUGE_ENUF))
  float f = INFINITY;
  

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