本文介绍了如何指定 setprecision 舍入的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
当流到标准输出时，我可以指定 setprecision 来舍入双精度值吗?
Can I specify setprecision to round double values when stream to std output?
ofile << std::setprecision(12) << total_run_time/TIME << "
";
输出:0.756247615801
Output: 0.756247615801
ofile << std::setprecision(6)<< total_run_time/TIME << "
";
输出:0.756248
但我需要输出为 0.756247
谢谢
推荐答案
还有std::fesetround，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 << "
";
// Restore previous rounding direction and output for testing:
std::fesetround(prev_round);
std::cout << "default: " << std::setprecision(6) << runtime << "
";
}
(请注意，这些不是我推荐的评论，它们只是用于辅导目的)
输出:
desired: 0.756247
default: 0.756248
重要提示:我没有在标准中发现任何关于浮动类型的 operator<< 重载有遵守舍入方向.
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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