如何指定setprecision舍入

Question

当流到std输出时,我可以指定setprecision来舍入double值吗？

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

输出: 0.756247615801

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

输出: 0.756248

但我需要输出为 0.756247

谢谢

Answer 1

还有std::fesetroundfrom <cfenv>,它设置了舍入方向:

#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,它们仅用于辅导目的)

输出:

desired: 0.756247
default: 0.756248

但重要提示:我没有在标准中找到任何提及,operator<<浮动类型的重载必须遵循舍入方向.