C编程-浮动条件时while循环的异常行为 [英] C Programming - Anomaly behaviour of while loop for float condition

问题描述

我编写了以下代码:

#include <stdio.h>

int main ()
{
  float x = 1.1;
  printf("%s\n", "Hello!");
  while (x == 1.1)
  {
    printf("%s\n", "Hey there!");
    printf("%f\n", x);
    x = x - 0.1;
  }
  printf("%s\n", "Bye!");
  return 0;
}

但是输出是(我认为这不是预期的):

However the output was (which I assume was not expected):

aps120797@XENON-PC:/mnt/d/Codes/LetUsC$ gcc C04Ag.c
aps120797@XENON-PC:/mnt/d/Codes/LetUsC$ ./a.out
Hello!
Bye!

要检查它是否接受浮动条件，我编写了以下代码:

To check if it accepts float condition or not, I wrote this code:

#include <stdio.h>

int main ()
{
  float x = 1.1;
  printf("%s\n", "Hello!");
  while (x >= 1.0)
  {
    printf("%s\n", "Hey there!");
    printf("%f\n", x);
    x = x - 0.1;
  }
  printf("%s\n", "Bye!");
  return 0;
}

我得到了预期的输出.

aps120797@XENON-PC:/mnt/d/Codes/LetUsC$ gcc C04Ag.c
aps120797@XENON-PC:/mnt/d/Codes/LetUsC$ ./a.out
Hello!
Hey there!
1.100000
Hey there!
1.000000
Bye!

所以，我的问题是，我在第一个代码中做错了什么?

So, my question is, what am I doing wrong in the first code?

更新:只是想出了解决此错误的方法. 附加了while条件，如下所示:while (x == 1.1f)

UPDTATE: Just figured out how to correct this error. Appended the while condition like this: while (x == 1.1f)

推荐答案

1.1不是float值，而是double值.

编写float x = 1.1;时，编译器将插入一个隐式强制转换:float x = (float)1.1;.

When you write float x = 1.1; the compiler inserts an implicit cast: float x = (float)1.1;.

编写x == 1.1时，编译器将插入另一个隐式强制转换:(double)x == 1.1.

When you write x == 1.1 the compiler inserts another implicit cast: (double)x == 1.1.

因此，有效地测试了将1.1强制转换为float并返回到double后1.1值是否仍然相同-即(double)(float)1.1 == 1.1是否为真.

So effectively you are testing whether 1.1 is still the same value after casting it to float and back to double - i.e. whether (double)(float)1.1 == 1.1 is true.

(double)(float)1.1 == 1.1不正确.至少在我的平台上:

(double)(float)1.1 == 1.1 is not true, due to floating-point rounding error. At least on my platform:

• 1.1实际上是1.100000000000000088817841970012523233890533447265625
• (double)(float)1.1实际上是1.10000002384185791015625

• 1.1 is actually 1.100000000000000088817841970012523233890533447265625
• (double)(float)1.1 is actually 1.10000002384185791015625

并且您会看到这两个数字不相同.

and as you can see these two numbers are not the same.

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