c - 什么时候a>a为真？

Question

是的，我想我真的是在做梦。我在 AIX 机器上编译并运行了以下代码:

AIX 3 5
PowerPC_POWER5 processor type
IBM XL C/C++ for AIX, V10.1
Version: 10.01.0000.0003


#include <stdio.h>
#include <math.h>

#define RADIAN(x) ((x) * acos(0.0) / 90.0)

double nearest_distance(double radius,double lon1, double lat1, double lon2, double lat2){
    double rlat1=RADIAN(lat1);
    double rlat2=RADIAN(lat2);
    double rlon1=lon1;
    double rlon2=lon2;
    double a=0,b=0,c=0;

    a = sin(rlat1)*sin(rlat2)+ cos(rlat1)*cos(rlat2)*cos(rlon2-rlon1);
    printf("%lf\n",a);
    if (a > 1) {
      printf("aaaaaaaaaaaaaaaa\n");
    }
    b = acos(a);
    c = radius * b;

    return radius*(acos(sin(rlat1)*sin(rlat2)+
        cos(rlat1)*cos(rlat2)*cos(rlon2-rlon1)));

}

int main(int argc, char** argv) {
  nearest_distance(6367.47,10,64,10,64);
  return 0;
}

现在，计算后“a”的值报告为“1”。而且，在这台 AIX 机器上，当我输入“if”时，看起来 1 > 1 为真!我认为“1”的 acos 返回 NanQ，因为 1 大于 1。请问这怎么可能？我不知道该怎么想了!

该代码在其他架构上运行得很好，其中“a”实际上取的是我认为的 1 的值，而 acos(a) 是 0。

Answer 1

If you do a comparison where result and expctedResult are float types:

if (result == expectedResult)

Then it is unlikely that the comparison will be true. If the comparison is true then it is probably unstable – tiny changes in the input values, compiler, or CPU may change the result and make the comparison be false.

与 epsilon 比较 – 绝对误差

if (fabs(result - expectedResult) < 0.00001)

来自Comparing floating point numbers

<小时/>

What Every Computer Scientist Should Know About Floating-Point Arithmetic