双精度和字符串流格式 [英] double and stringstream formatting

问题描述

double val = 0.1;
std::stringstream ss;
ss << val;
std::string strVal= ss.str();

在Visual Studio调试器中， val 具有值0.10000000000000001（因为无法表示0.1）。
使用字符串流转换 val 时， strVal 等于 0.1 。但是，使用boost :: lexical_cast时，生成的 strVal 为 0.10000000000000001 。

In the Visual Studio debugger, val has the value 0.10000000000000001 (because 0.1 can't be represented). When val is converted using stringstream, strVal is equal to "0.1". However, when using boost::lexical_cast, the resulting strVal is "0.10000000000000001".

另一个示例如下：

double val = 12.12305000012;

在视觉工作室 val 下显示为12.123050000119999，并使用stringstream和默认精度（6）变为12.1231。我不太明白为什么不是12.12305（...）。

Under visual studio val appears as 12.123050000119999, and using stringstream and default precision (6) it becomes 12.1231. I don't really understand why it is not 12.12305(...).

是否存在默认精度，或者stringstream是否具有特定的算法来转换双精度值

Is there a default precision, or does stringstream have a particular algorithm to convert a double value which can't be exactly represented?

谢谢。

推荐答案

您可以如下更改 stringstream 的浮点精度：

You can change the floating-point precision of a stringstream as follows:

double num = 2.25149;
std::stringstream ss(stringstream::in | stringstream::out);
ss << std::setprecision(5) << num << endl;
ss << std::setprecision(4) << num << endl;

输出：

2.2515
2.251

请注意在适当的时候如何对数字进行四舍五入。

Note how the numbers are also rounded when appropriate.

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