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浮点数0.5f的偶数舍入运算实现

来源：花匠小妙招时间：2024-12-03 05:28

深入理解计算机系统家庭作业 2.93

最新推荐文章于 2021-05-18 00:34:48 发布

fengjingge 于 2014-04-28 13:07:05 发布

实现浮点数 0.5f,运行成功，特别是要注意舍入运算，是偶数舍入！

#include <stdio.h>
typedef unsigned float_bits;

typedef unsigned char *byte_pointer;
void show_bytes(byte_pointer start,int len)
{
   int i;
   for(i = 0;i < len;i++)
   {
       printf(" %.2x",start[i]);
   }
   printf("n");
}

void show_float(float x)
{
show_bytes((byte_pointer) &x,sizeof(float));
}

float_bits float_half(float_bits f)
{
        unsigned sign = f>>31;
        unsigned exp = f>>23&0xff;
        unsigned frac = f&0x7fffff;
        if(exp == 0xff)
                return f;
        if(exp == 0)
   {
       if ((frac & 0x00000001) == 1)
           {
               if ((frac & 0x00000002) == 0x00000002)
                   frac = (frac + 1) >> 1;
               else
                   frac = frac >> 1;
           }
       else
           {
               frac = frac >> 1;
           }
   }
        if(exp == 1)
        {
                exp = exp - 1;
                 if ((frac & 0x00000001) == 1)
                          {
                                  if ((frac & 0x00000002) == 0x00000002)
                                          frac = (frac + 1) >> 1;
                                  else
                                          frac = frac >> 1;
                          }
                  else
                          {
                                  frac = frac >> 1;
                          }
                frac = frac + (1 << 22);
        }
        if(exp > 1)
        {
                exp = exp - 1;
        }
        return (sign<<31) | (exp<<23) | frac;
}
int main()
{

unsigned u,ur;
float f,fr;

    for(u=8388607;u<=0x0fffffffu;u++)
    {
        f =*((float *)&u);
        f=f*0.5;
        ur = float_half(u);
        fr = *((float *)&ur);
        if(fr == f)
            printf("%u: %x %x okn",u,f,fr);
        else
        {
            printf("%u: errorn",u);
        printf("the f is n");
        show_float(f);
        printf("th fr is %n");
        show_float(fr);
            printf("%x %xn",f,fr);
            return -1;
        }
     }
   return 0;
}