C / C ++浮点问题 [英] C/C++ floating point issue

问题描述

我一个基本的浮点precision问题挣扎。这里的问题是：

I am struggling with a basic floating-point precision issue. Here is the problem:

double d = 0.1;
d += 0.1;
d += 0.1;

d == 0.3 ? std::cout << "yes" : std::cout << "no";

运行code，你会得到没有

Run the code and you get "no"

据我了解，在二进制和二进制存储C / C ++存储值不能准确存储每个值。我也明白，这些小错误化合物你他们做各种数学运算（即D + = 0.1;）

I understand that C/C++ store values in binary and that binary storage can not exactly store every value. I also understand that these small errors compound as you do various math operations on them (i.e. d += 0.1;).

我的问题是，如果我需要测试当d = = 0.3（一个合理的precision ..因为是code的明确意图以上）......我该怎么办呢？我希望答案是不是：

My questions is if I do need to test if d == 0.3 (to a reasonable precision.. as is the clear intent of code above)... how do I do that? I hope the answer is not:

if (d > 0.2999 && d < 0.3001) ...

还..这个作品

float f = 0.1;
f += 0.1;
f += 0.1;

f == 0.3f ? std::cout << "yes" : std::cout << "no";

但我能找到的语言中没有对等0.3D。

but I can find no equivalent "0.3d" in the language.

感谢

推荐答案

测试平等一定的耐受度的一种常用方法是使用的 晶圆厂 ：

One common way of testing equality to a certain "tolerance level" is using fabs:

if (fabs(d - 0.3) < 1E-8) {
     ...
}

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