奇怪的双到int转换行为在c ++ [英] strange double to int conversion behavior in c++

问题描述

以下程序显示了我在c ++中看到的奇怪的双到int转换行为：

The following program shows the weird double to int conversion behavior I'm seeing in c++:

#include <stdlib.h>
#include <stdio.h>

int main() {
  double d = 33222.221;
  printf("d = %9.9g\n",d);

  d *= 1000;
  int i = (int)d;

  printf("d = %9.9g | i = %d\n",d,i);

  return 0;
}

当我编译和运行程序时，我看到：

When I compile and run the program, I see:

g++ test.cpp
./a.out
d = 33222.221
d =  33222221 | i = 33222220

为什么i不等于33222221？
编译器版本是GCC 4.3.0

Why is i not equal to 33222221? The compiler version is GCC 4.3.0

推荐答案

浮点表示几乎从不精确（仅在特殊情况下） 。每个程序员都应该阅读：每个计算机科学家应该知道的浮点算术

Floating point representation is almost never precise (only in special cases). Every programmer should read this: What Every Computer Scientist Should Know About Floating-Point Arithmetic

简而言之 - 您的电话号码大概是33222220.9999999999999999999999999999999999999999999999999999999999998（或类似内容），在截断后会变成33222220。

In short - your number is probably 33222220.99999999999999999999999999999999999999999999999999999999999999998 (or something like that), which becomes 33222220 after truncation.

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