如何指定精度舍入 [英] How to specify setprecision rounding

问题描述

当流到std输出时，我是否可以指定setprecision以舍入双精度值？

Can I specify setprecision to round double values when stream to std output?

ofile << std::setprecision(12) << total_run_time/TIME << "\n";

输出：
0.756247615801

Output: 0.756247615801

ofile << std::setprecision(6)<< total_run_time/TIME << "\n";

输出：
0.756248

Output: 0.756248

但我需要输出 0.756247

感谢

推荐答案

从< cfenv>还有 std :: fesetround ，它设置舍入方向：

There is also std::fesetround from <cfenv>, which sets the rounding direction:

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cfenv>

int main () {
    double runtime = 0.756247615801;

    // Set rounding direction and output with some precision:
    const auto prev_round = std::fegetround();
    std::fesetround(FE_DOWNWARD);    
    std::cout << "desired: " << std::setprecision(6) << runtime << "\n";

    // Restore previous rounding direction and output for testing:
    std::fesetround(prev_round);
    std::cout << "default: " << std::setprecision(6) << runtime << "\n";
}

（请注意，这些不是我推荐的评论，他们只是为了辅导目的）

输出：

desired: 0.756247
default: 0.756248

/ strong>我没有在标准中找到任何提及，运算符<< 浮动类型的重载有来遵守舍入方向。

Important note, though: I did not find any mention in the standard, that the operator<< overloads for floating types have to honour the rounding direction.

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